Well, if it's a big school and only 3/4ths of the classrooms are used at any time it could save a few classrooms. It shouldn't take long for authorities to arrive and of course the stupid incel won't last long against trained gunslingers. Also, school shooters aren't exactly known for being able to kick down doors. I guess the best they could do is smash the little window and try to do a corner shot, but without a cornershot.