Illya did look a little old for a college student, on the show. But the beauty of fan fiction can eliminate that problem as we can make him look as youthful as we want in our imaginations.

Going back to school, It might have been hard for either of them to do that given they would get minimal notice to head out on an assignment. There'd be an awful lot of makeup classes and tests, I would imagine.

Maybe as their careers they might have been able to try, or even just think about it.

Given the occupations they had for the reunion movie, I don't think either of them furthered their education. After all they learned enough from the 'school of hard knocks.'