In American English:
Mark is in college. = Mark is currently attending a college/university.
Mark is at college. (Some people use this to mean the same thing as the first sentence.)
I think most Americans would use the word college no matter whether the particular school is actually called "college" or "university" (i.e. Swarthmore College / Princeton University)
Using a specific name, I think most people would then use "at":
Mark is at Princeton.
Mark is at Swarthmore.
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