Bullet proof glass in schools

I had an idea. There were security measures in place in the school in CT but the gunman still got in. OK, how about this?Continue to Keep doors locked and keep intercom system in place, replace all outside windows and doors with bullet proof glass,have metal detectors at door entrance(S) with a couple armed guards at the door to do a search for what can not be detected through the metal detector. This way people wont have to worry about guns in the class rooms.