kindergarten

One would say "he is in kindergarten" if one were describing what level he attends in the educational system. One would say "he is at kindergarten" if one were describing where he is at the moment, his physical location.

Also WB, "at" might imply "about", as in "he is working at at economics"

We'd be more likely to say "at the university" or "at university level"

Thank you`