Upsert up to 1000 groups in a single request. Matches by externalReferenceId or groupId within school scope. Returns 200 when all items succeed, 207 for partial success with per-item failures.

Per-item error codes: VALIDATION_ERROR, AMBIGUOUS_GROUP_IDENTIFIER, AMBIGUOUS_PARENT_GROUP_IDENTIFIER, GROUP_NOT_FOUND, DUPLICATE_IN_REQUEST, PARENT_GROUP_NOT_FOUND, ARCHIVED_GROUP_EXISTS, PARENT_CREATION_FAILED, CIRCULAR_HIERARCHY, CREATE_FAILED, UPDATE_FAILED.