The third prong is a bit badly posed: descriptively, white kids test better than black kids, and each of the three prongs offers an explanation. The third prong points to a discredited belief of genetic inferiority; by positioning the three prongs as exhaustive, the author structures the argument such that if you don't accept either of the first two prongs, then you must be a racist.

But you mentioned that a test was taken. Is the test somehow unscientific? Is it rigged to favor white kids? Are you speaking of a hypothetical test that doesn't exist and was never applied?

If a test was actually taken, and it is not rigged, how can it not be a sort of scientific evidence?