Or overcooked ramen. It's sticky and be thrown at the shooters face to blind him. Or better yet every kid gets a can of silly string...
once the shooter is covered in silly string the children could scurry to safety.
Plus teachers could then be trained on how to takedown a silly string covered shooter and restrain them until police arrive. Of course that would be voluntary training, perhaps a small bonus for teachers that can show proficiency at taking down the shooter without themselves becoming entangled in the silly string.