in school? at school?

Hi!

I've always thought that one says at school and not in school ... However, this sentence comes from Longman Dictionary of Contemporary English:

"Sandra's not in school today"

Would there by a difference in meaning if I said at school rather than in school ?

thanks, best wishes,

 Both prepositions are correct. In is used quite a lot in American English: What did you learn at/in school today?

CB