Well, they reason, guns are responsible for much of the violence in our society today, so a ban on guns would reduce the violence. In the absence of guns, Bad Guys will choose other means of expressing their violent rage, be it knives, bombs, cars, chopsticks, or stones. Violence cannot be legislated away. Despite all the laws that require background checks, waiting periods, firearms training, or that simply prohibit firearms carry by private citizens, more and more people are getting shot at in public places. Bad Guys know that places where guns are prohibited provide a target-rich environment, and, more importantly, an environment where it's unlikely that they'll be facing return fire.
Public schools are especially attractive, in part because of the ratio of young people to adults.
Students in schools are under increasing surveillance, including locker and vehicle searches, and metal detector checks upon entering the school in some places. A young person who wants an education won't risk expulsion, so it's highly unlikely that most kids will carry a weapon for protection. Teachers and school administrators generally don't have to go through that gauntlet, but the laws apply to them as well. If they ignore the laws, they can be fired, and with the horror of carrying a firearm to school on their record, it's a foregone conclusion that no other school district would hire such a monster. Yet that's what is happening with gun control laws.