They don't want the students fighting back because a student might get hurt and the school might be held liable.
There is the ugly truth. It's all CYA. If the kid had succeeded in blowing his classmate's head off the school would have talked about how very tragic it was and many people would say how sad it is. I'm sure some gun grabbers would pop up and wave around a bloody shirt.
However, because someone actually confronted the shooter instead of the school's preferred run and hide method (regardless of the fact there was nowhere to run or hide) the administration is flipping out because an injury could land them in a lawsuit.
Plus, schools in general teach non-confrontation and have done it forever. I had a kid pin me to a desk in 5th grade and started punching my lights out. After 3 solid hits and recovering from the initial shock I punched back hard enough to make him reel and give me a chance to get my bearings. A teacher stepped in and broke it up. We both went to the principal's office.
The principal admonished me for not removing myself from the situation. This was almost 3 decades ago before the depths of PC had firmly clutched our schools. The trend has been going on for awhile. "Violence is never the answer" is a common creed among teachers.