I don't think you'll find a consistent answer with this one.
You can take a course at the local college.
He took a course in the local college.
He is at college today (means he is attending college today, either as a student or a teacher).
He is in the college at the moment (this just refers to his physical location, not to what he's doing there).
The last two examples above can be used as a "rule" if you like as far as the meaning goes, but "in" and "at" are used in different ways too, to which that "rule" might not apply.
There's a difference between BrE and AmE too.
She's at school. BrE.
She's in school. AmE.



Reply With Quote

