In BrE either of your suggestions would be fine. (OK) (not ok) We would say in just as readily as at. Perhaps that's because school is usually indoors.
"I'm at school" could mean I am a school-age person, or I have just arrived off the bus.
"I'm in school" could mean I am not absent today, or it could mean I'm not on the playing field.
The possibilities are endless and we would use both.
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