Just the knowledge that even one teacher at a school might be armed would be the deterrent we don't have now. The people doing this are cowards. Well, in fact this type of thing has happened before. Ditto for rapes and armed assaults. States with concealed carry have seen their crime rates in these areas drop like a rock.
Criminals are fearful of being shot before they can carry out a crime.
Concealed carry translates into few crimes. Guns in the hands of honest people actually save lives since not too many criminals are ever shot and even fewer honest people are maimed and killed. Back then the federal government solved the problem with firearms. It wasn't long before these people had shot a few hijackers. And with the downings of just a handful of these criminals by armed marshals or snipers, there was a quick end to the nightly news reports of hijackings.