“in school” refers to students learning. "at school" can be used for students doing anything that isn't learning; it can also be used for teachers teaching. "At a school" is used for anything else. Examples: In school, I learned that Columbus came to America in 1492. In school, I learned that F=MA, but the situation in the real world is more complex. Bob was mean to me at school today! Sally, behave! You can do that at home, but you can't do it at school! At school, Teacher John is very strict, but if you meet him anywhere else, he's nice. I thought I followed the directions to your house, but now I'm at a school. Where did I make I wrong turn? How can I get to your house? Bob works as a janitor at a school.