If you get a visitors pass and have a legitimate reason to be there no one will say anything. You can get a visitors pass and go into your old teacher's classrooms and say yo, people do that here all the time and the teacher has a 5 minute convo with them about how they've grown and then we go back to the lesson. Anyway, if you're just standing around someone might ask if you need help and if you say "Nope, just standing" there might be an issue. You wouldn't be arrested but you'd be escorted off of campus unless you were deemed a threat in which case the school would go into lockdown and you'd feel like the biggest starfish.