diff --git a/mesa/agent.py b/mesa/agent.py
index 2b6d2f99343..18454164e5a 100644
--- a/mesa/agent.py
+++ b/mesa/agent.py
@@ -62,9 +62,9 @@ def __init__(self, model: Model, *args, **kwargs) -> None:
         super().__init__(*args, **kwargs)
 
         self.model: Model = model
-        self.model.register_agent(self)
         self.unique_id: int = next(self._ids[model])
         self.pos: Position | None = None
+        self.model.register_agent(self)
 
     def remove(self) -> None:
         """Remove and delete the agent from the model."""
diff --git a/tests/test_time.py b/tests/test_time.py
new file mode 100644
index 00000000000..f180ac25465
--- /dev/null
+++ b/tests/test_time.py
@@ -0,0 +1,317 @@
+"""Test the advanced schedulers."""
+
+import unittest
+from unittest import TestCase, mock
+
+from mesa.agent import Agent
+from mesa.model import Model
+from mesa.time import (
+    BaseScheduler,
+    RandomActivation,
+    RandomActivationByType,
+    SimultaneousActivation,
+    StagedActivation,
+)
+
+RANDOM = "random"
+STAGED = "staged"
+SIMULTANEOUS = "simultaneous"
+RANDOM_BY_TYPE = "random_by_type"
+
+
+class MockAgent(Agent):
+    """Minimalistic agent for testing purposes."""
+
+    def __init__(self, model):  # noqa: D107
+        super().__init__(model)
+        self.steps = 0
+        self.advances = 0
+
+    def kill_other_agent(self):  # noqa: D102
+        for agent in self.model.schedule.agents:
+            if agent is not self:
+                agent.remove()
+
+    def stage_one(self):  # noqa: D102
+        if self.model.enable_kill_other_agent:
+            self.kill_other_agent()
+        self.model.log.append(f"{self.unique_id}_1")
+
+    def stage_two(self):  # noqa: D102
+        self.model.log.append(f"{self.unique_id}_2")
+
+    def advance(self):  # noqa: D102
+        self.advances += 1
+
+    def step(self):  # noqa: D102
+        if self.model.enable_kill_other_agent:
+            self.kill_other_agent()
+        self.steps += 1
+        self.model.log.append(self.unique_id)
+
+
+class MockModel(Model):  # Noqa: D101
+    def __init__(
+        self, shuffle=False, activation=STAGED, enable_kill_other_agent=False, seed=None
+    ):
+        """Creates a Model instance with a schedule.
+
+        Args:
+            shuffle (Bool): whether or not to instantiate a scheduler
+                            with shuffling.
+                            This option is only used for
+                            StagedActivation schedulers.
+
+            activation (str): which kind of scheduler to use.
+                              'random' creates a RandomActivation scheduler.
+                              'staged' creates a StagedActivation scheduler.
+                              The default scheduler is a BaseScheduler.
+            enable_kill_other_agent: whether or not to enable kill_other_agent
+            seed : rng
+        """
+        super().__init__(seed=seed)
+        self.log = []
+        self.enable_kill_other_agent = enable_kill_other_agent
+
+        # Make scheduler
+        if activation == STAGED:
+            model_stages = ["stage_one", "model.model_stage", "stage_two"]
+            self.schedule = StagedActivation(
+                self, stage_list=model_stages, shuffle=shuffle
+            )
+        elif activation == RANDOM:
+            self.schedule = RandomActivation(self)
+        elif activation == SIMULTANEOUS:
+            self.schedule = SimultaneousActivation(self)
+        elif activation == RANDOM_BY_TYPE:
+            self.schedule = RandomActivationByType(self)
+        else:
+            self.schedule = BaseScheduler(self)
+
+        # Make agents
+        for _ in range(2):
+            agent = MockAgent(self)
+            self.schedule.add(agent)
+
+    def step(self):  # noqa: D102
+        self.schedule.step()
+
+    def model_stage(self):  # noqa: D102
+        self.log.append("model_stage")
+
+
+class TestStagedActivation(TestCase):
+    """Test the staged activation."""
+
+    expected_output = ["1_1", "1_1", "model_stage", "1_2", "1_2"]
+
+    def test_no_shuffle(self):
+        """Testing the staged activation without shuffling."""
+        model = MockModel(shuffle=False)
+        model.step()
+        model.step()
+        assert all(i == j for i, j in zip(model.log[:5], model.log[5:]))
+
+    def test_shuffle(self):
+        """Test the staged activation with shuffling."""
+        model = MockModel(shuffle=True)
+        model.step()
+        for output in self.expected_output[:2]:
+            assert output in model.log[:2]
+        for output in self.expected_output[3:]:
+            assert output in model.log[3:]
+        assert self.expected_output[2] == model.log[2]
+
+    def test_shuffle_shuffles_agents(self):  # noqa: D102
+        model = MockModel(shuffle=True)
+        a = mock.Mock()
+        model.schedule._agents.random = a
+        assert a.shuffle.call_count == 0
+        model.step()
+        assert a.shuffle.call_count == 1
+
+    def test_remove(self):
+        """Test the staged activation can remove an agent."""
+        model = MockModel(shuffle=True)
+        agents = list(model.schedule._agents)
+        agent = agents[0]
+        model.schedule.remove(agents[0])
+        assert agent not in model.schedule.agents
+
+    def test_intrastep_remove(self):
+        """Test the staged activation can remove an agent in a step of another agent.
+
+        so that the one removed doesn't step.
+        """
+        model = MockModel(shuffle=True, enable_kill_other_agent=True)
+        model.step()
+        assert len(model.log) == 3
+
+    def test_add_existing_agent(self):  # noqa: D102
+        model = MockModel()
+        agent = model.schedule.agents[0]
+        with self.assertRaises(Exception):
+            model.schedule.add(agent)
+
+
+class TestRandomActivation(TestCase):
+    """Test the random activation."""
+
+    def test_init(self):  # noqa: D102
+        model = Model()
+        agents = [MockAgent(model) for _ in range(10)]
+
+        scheduler = RandomActivation(model, agents)
+        assert all(agent in scheduler.agents for agent in agents)
+
+    def test_random_activation_step_shuffles(self):
+        """Test the random activation step."""
+        model = MockModel(activation=RANDOM)
+        a = mock.Mock()
+        model.schedule._agents.random = a
+        model.schedule.step()
+        assert a.shuffle.call_count == 1
+
+    def test_random_activation_step_increments_step_and_time_counts(self):
+        """Test the random activation step increments step and time counts."""
+        model = MockModel(activation=RANDOM)
+        assert model.schedule.steps == 0
+        assert model.schedule.time == 0
+        model.schedule.step()
+        assert model.schedule.steps == 1
+        assert model.schedule.time == 1
+
+    def test_random_activation_step_steps_each_agent(self):
+        """Test the random activation step causes each agent to step."""
+        model = MockModel(activation=RANDOM)
+        model.step()
+        agent_steps = [i.steps for i in model.schedule.agents]
+        # one step for each of 2 agents
+        assert all(x == 1 for x in agent_steps)
+
+    def test_intrastep_remove(self):
+        """Test the random activation can remove an agent in a step of another agent.
+
+        so that the one removed doesn't step.
+        """
+        model = MockModel(activation=RANDOM, enable_kill_other_agent=True)
+        model.step()
+        assert len(model.log) == 1
+
+    def test_get_agent_keys(self):  # noqa: D102
+        model = MockModel(activation=RANDOM)
+
+        keys = model.schedule.get_agent_keys()
+        agent_ids = [agent.unique_id for agent in model.agents]
+        assert all(entry_i == entry_j for entry_i, entry_j in zip(keys, agent_ids))
+
+        keys = model.schedule.get_agent_keys(shuffle=True)
+        agent_ids = {agent.unique_id for agent in model.agents}
+        assert all(entry in agent_ids for entry in keys)
+
+    def test_not_sequential(self):  # noqa: D102
+        model = MockModel(activation=RANDOM)
+        # Create 10 agents
+        for _ in range(10):
+            model.schedule.add(MockAgent(model))
+        # Run 3 steps
+        for _ in range(3):
+            model.step()
+        # Filter out non-integer elements from the log
+        filtered_log = [item for item in model.log if isinstance(item, int)]
+
+        # Check that there are no 18 consecutive agents id's in the filtered log
+        total_agents = 10
+        assert not any(
+            all(
+                (filtered_log[(i + j) % total_agents] - filtered_log[i]) % total_agents
+                == j % total_agents
+                for j in range(18)
+            )
+            for i in range(len(filtered_log))
+        ), f"Agents are activated sequentially:\n{filtered_log}"
+
+
+class TestSimultaneousActivation(TestCase):
+    """Test the simultaneous activation."""
+
+    def test_simultaneous_activation_step_steps_and_advances_each_agent(self):
+        """Test the simultaneous activation step causes each agent to step."""
+        model = MockModel(activation=SIMULTANEOUS)
+        model.step()
+        # one step for each of 2 agents
+        agent_steps = [i.steps for i in model.schedule.agents]
+        agent_advances = [i.advances for i in model.schedule.agents]
+        assert all(x == 1 for x in agent_steps)
+        assert all(x == 1 for x in agent_advances)
+
+
+class TestRandomActivationByType(TestCase):
+    """Test the random activation by type.
+
+    TODO implement at least 2 types of agents, and test that step_type only
+    does step for one type of agents, not the entire agents.
+    """
+
+    def test_init(self):  # noqa: D102
+        model = Model()
+        agents = [MockAgent(model) for _ in range(10)]
+        agents += [Agent(model) for _ in range(10)]
+
+        scheduler = RandomActivationByType(model, agents)
+        assert all(agent in scheduler.agents for agent in agents)
+
+    def test_random_activation_step_shuffles(self):
+        """Test the random activation by type step."""
+        model = MockModel(activation=RANDOM_BY_TYPE)
+        a = mock.Mock()
+        model.random = a
+        for agentset in model.schedule._agents_by_type.values():
+            agentset.random = a
+        model.schedule.step()
+        assert a.shuffle.call_count == 2
+
+    def test_random_activation_step_increments_step_and_time_counts(self):
+        """Test the random activation by type step increments step and time counts."""
+        model = MockModel(activation=RANDOM_BY_TYPE)
+        assert model.schedule.steps == 0
+        assert model.schedule.time == 0
+        model.schedule.step()
+        assert model.schedule.steps == 1
+        assert model.schedule.time == 1
+
+    def test_random_activation_step_steps_each_agent(self):
+        """Test the random activation by type step causes each agent to step."""
+        model = MockModel(activation=RANDOM_BY_TYPE)
+        model.step()
+        agent_steps = [i.steps for i in model.schedule.agents]
+        # one step for each of 2 agents
+        assert all(x == 1 for x in agent_steps)
+
+    def test_random_activation_counts(self):
+        """Test the random activation by type step causes each agent to step."""
+        model = MockModel(activation=RANDOM_BY_TYPE)
+
+        agent_types = model.agent_types
+        for agent_type in agent_types:
+            assert model.schedule.get_type_count(agent_type) == len(
+                model.agents_by_type[agent_type]
+            )
+
+    # def test_add_non_unique_ids(self):
+    #     """
+    #     Test that adding agent with duplicate ids result in an error.
+    #     TODO: we need to run this test on all schedulers, not just
+    #     TODO:: identical IDs is something for the agent, not the scheduler and should be tested there
+    #     RandomActivationByType.
+    #     """
+    #     model = MockModel(activation=RANDOM_BY_TYPE)
+    #     a = MockAgent(0, model)
+    #     b = MockAgent(0, model)
+    #     model.schedule.add(a)
+    #     with self.assertRaises(Exception):
+    #         model.schedule.add(b)
+
+
+if __name__ == "__main__":
+    unittest.main()