Quite the contrary, there aren't enough guns. If the teachers were carrying, if the school had armed security guards, if the principal had a handgun, etc. then this "crazy" person would likely not have acted out. As Quannel X says, a person may be "crazy" but that doesn't mean they're crazy. That is, most persons who are classified as mentally ill don't act out in a situation where they will likely lose.
"...please explain why a school teacher would have three guns in her house, one of them a Glock semi-automatic pistol?"
Perhaps because it is allowed, because she enjoys target shooting, because she hunts and the .223 semi-auto AR-15 is one of the most popular hunting rifles in existence in the USA. Because the Glock is an OK and very popular handgun.
The question for the mother is not why she owned firearms but why she allowed her son unsupervised access to her firearms. She made a mistake and paid the price. Darwin rulez once again.