Hmmm let's think a bit. Shooter in the hallway. Teacher rushes out and fires 5 shots at the shooter. Statistics show that even well trained police, in a shootout, hit the target about 18% of the time. 1 in 5. So said teacher shoots 5 and luckily hits the shooter. 4 bullets miss. Crowded hallway. Statistics also indicate a shooter hits someone about 40-50% of the time when firing randomly (its an almost can't miss situation).
Do the math. The teacher becomes the new shooter. He/she has 4 directed but inaccurate shots in the crowded hallway. Ya' follow? There is every chance that 1 or 2 of his misses will "hit" someone else in the hallway...averages being what they are.
WHEN YOU ADD SHOOTERS TO AN ATTACK YOU ADD BULLETS WITH ONLY A SMALL CHANCE OF SUCCESS AND A GREAT CHANCE OF UNINTENTIONAL HARM.
Assuming of course that the "trained" teacher has the moxie when crunch time comes to rush out with pistol to confront a military rifle....and that he shoots as well as a trained policeman.
Second, this would be a bonanza for attorneys who will have no end to litigation opportunities right down to careless display of a firearm in class, or God forbid that a student figures out how to sneak it out of its hiding place....and you know that they will try.
How about taking away the assault rifles? Then there is a greatly reduced argument. Baddabing Baddaboom. Issue solved.
Ya' follow?