The "disparate impact" test applies mainly in civil rights act litigation, and it's extremely hard to make a case under that theory. It's a three-part test, and showing that a particular policy has a disparate impact on one race is just the first hurdle.

You also have to prove that the party acted with malice: either the policy exists for explicitly racist reasons, or the race-neutral justification is pretextual. If you can do that, you _also_ have to prove that there is a less-discriminatory alternative policy that achieves the same goal.

see, e.g., https://www.justice.gov/crt/fcs/T6Manual7#C'

It is definitely not de facto illegal to have a racially lopsided student body -- the school might be asked to justify the specific policy or practice that led to that outcome on race-neutral grounds, but saying "GPA and test scores" would be more than enough.