The fact that mangas in general see how casually is to do trespassing in school bothers me to no end.
Don't know where you are from. And it's probably partly also school-specific. But at least my school was open quite late because there were a library, some hobby activities etc. Of course it wasn't open in the middle of the night. But I knew (and I wasn't the only one) how to get in there even at night easily if I wanted to.