Most pepper spray products are useless against attackers with firearms, because long before a defender is in range to use it, he/she is a corpse. OTOH, there have been improvements in paint-gun pepper pellets that look effective now -- something I'd forgotten. Assuming they work in actual paint guns and store well, hanging one in each room of a school would make for an effective defense.
The fun of it is that teachers could train in their use by using ordinary paint ball guns in activities with students.
I'd have to do some research, but this is a possible route for schools. It counts as arming teachers, but rather than blood it would just leave light orange spots, and no holes in the facilities. On top of those, the headlines would be depressing to would-be mass shooters:
SCHOOL LIBRARIAN STOPS SHOOTER WITH PAINT BALL GUN.
I'd call that a serious psychological deterrent.
edit: I just remembered one problem with pepper spray -- if the target is wearing contact lenses, its effectiveness is reduced drastically.