It would be a "diagnostic test." A "diagnosis" is the result of a diagnostic test. For example, a diagnostic test might be to take a blood sample and test it in a lab. If the test showed high levels of thyroid hormone, the diagnosis could be Graves' disease.
Although if you were evaluating a student for a disability or learning disorder, or even just to determine proficiency in a subject like reading, that could also be a diagnostic test. My wife did educational research for a long time and they used that term for some of the tests they administered. I guess in general I would say that "diagnostic test" implies that you think something may be wrong and are testing for it (usually in a medical or psychological sense). It also has a more scientific or formal connotation.