Because if your stupid like you might be you stick it under desks
Every school can make their own policy on that issue. Most schools allow you to chew gum unless it has created a problem with clean up issues or distractions in the class room.
I wish I had your school. All you hear all day at mine is people snapping it. So loud the whole hallway can hear. It makes me want to punch someone.
Because you're stupid and you stick it under chairs and desks