Both are correct. In this case it doesn't matter if you use "at" or "in".

两者都是正确的 "I'll see you at school" = when I get TO the location of school, I will see you there. "I'll see you in school" = when I go INSIDE the school, I will see you there . at school- i will see you there in school- i will see you inside there i dont know, hope this helps.