Create or update multiple students in a single request with partial success support. Uses externalReferenceId as the unique match key. Min 1, max 1000 students per request. Returns 200 when all items succeed, 207 for partial success with per-item failures.

Per-item error codes: VALIDATION_ERROR, DUPLICATE_EXTERNAL_REFERENCE_ID, REQUIRED_FIELD_MISSING, EMAIL_REQUIRED_FOR_CREDENTIALS, EXTERNAL_REF_ID_REQUIRED_WITH_EXTERNAL_REFERENCE_TYPE, USERNAME_COLLISION, STUDENT_IS_ARCHIVED, CREATE_FAILED, UPDATE_FAILED.