fix(api): add Submissions.unsubmit() to clear submitted_at with explicit null

campus_python/api/v1/submissions.py

@@ -178,6 +178,19 @@ def submit(self) -> None:
             resp.raise_for_status()
             return None
 
+        def unsubmit(self) -> None:
+            """Clear this submission's submitted_at timestamp.
+
+            The server only clears submitted_at for an explicit null in
+            the PATCH body; update() cannot express that because None
+            means "omit the field" there.
+            """
+            resp = self.client.patch(
+                self.make_path(), json={"submitted_at": None}
+            )
+            resp.raise_for_status()
+            return None
+
         class Responses(Resource):
             """Campus API Submission Responses resource."""
             path = "responses"

tests/unit/test_submissions_unsubmit.py

@@ -0,0 +1,52 @@
+"""Regression tests for the submissions unsubmit contract (issue #74).
+
+The server clears a submission's submitted_at only when the PATCH body
+carries an explicit null (campus/api/resources/submission.py), which is
+how campus-classroom submits its unsubmit. update() cannot express
+that — its None means "omit the field" — so Submissions.unsubmit()
+sends the explicit-null body itself.
+"""
+
+import unittest
+from unittest.mock import Mock
+
+from campus_python.api.v1 import ApiRoot
+
+
+def make_api() -> tuple[ApiRoot, Mock]:
+    """Create an ApiRoot backed by a mock JSON client."""
+    client = Mock()
+    return ApiRoot(json_client=client), client
+
+
+class TestSubmissionsUnsubmit(unittest.TestCase):
+    """unsubmit() must PATCH an explicit submitted_at null."""
+
+    def setUp(self):
+        self.api, self.client = make_api()
+
+    def test_unsubmit_patches_explicit_null(self):
+        self.api.submissions["sub-1"].unsubmit()
+        self.client.patch.assert_called_once_with(
+            "/api/v1/submissions/sub-1/",
+            json={"submitted_at": None},
+        )
+
+    def test_update_still_requires_a_field(self):
+        """update() keeps its "at least one field" contract."""
+        with self.assertRaises(ValueError):
+            self.api.submissions["sub-1"].update()
+        self.client.patch.assert_not_called()
+
+    def test_update_still_sets_timestamp(self):
+        self.api.submissions["sub-1"].update(
+            submitted_at="2026-10-04T01:02:03Z"
+        )
+        self.client.patch.assert_called_once_with(
+            "/api/v1/submissions/sub-1/",
+            json={"submitted_at": "2026-10-04T01:02:03Z"},
+        )
+
+
+if __name__ == "__main__":
+    unittest.main()