The ONLY thing that would make sense in a school is to have a trained, armed security officer that has a ton of active shooter training. Arming and training all of those teachers would cost way more than just hiring one or two well trained officers.
Even if a teach chose that route, it's only a matter of time before he/she shoots a kid by mistake thinking their life is in danger or something. That's just way too many underpaid professionals with weapons and hormonal teenagers/children. Fuck that noise.