Perhaps it's better to have the armed person be objective when it comes to issues with the kids. Maybe a teacher isn't the best person for that role because they interact with the kids. That's why I say security guards. Keep them out out sight and don't let the kids even really ever see them. Put'em in a room somewhere with a camera and the means for defense if they spot something going on.
Lunatics seem to be attracted to attacking children for whatever reason, so some type of security is in order. Also it creates a lot of security jobs that a monkey can do.