In this sentence is "at school":
Everyone at school knew he was the new kid on the block.
In this sentence is "in school":
When I was in school I would go to the library day to day to study.
Can you teach me about that difference.
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Prepositions are hard. Any rule about prepositions is plagued with exceptions.
"At" usually introduces the idea of a place or a time
At the park
"In" usually introduces the idea of containment.
Put the milk in the refrigerator
Come in out of the rain.
A school is both a building and a place so either preposition may be appropriate. I at least do not see any shade of nuance between using one or the other in this specific context.
Prepositions of place are a nightmare for learners so I will limit this to your specific question. Generally, whatever happens within the perimeter a particular institution can be said to happen 'in' or 'at' that place. So, in that sense they are interchangeable. 'At' may also refer to events that happen outside the perimeter, such as the area just outside the gates. You can ask your driver to drop you off at school or work etc. meaning on the street outside. My response is woefully incomplete but I hope you get the general idea.
"At school means the person is literally, physically, inside the school. ... In school means the person is studying in general (usually at college or university) but not necessarily inside the school building at that moment. “My husband doesn't have a job because he's in school."
End of quote.
Source: Espresso English