Assuming you're age is near my own, in our time, we never took guns to school. Yes, we guys did carry pocket knives, but they were never taken out for anything other than cleaning our fingernails or carving the name of this weeks love of our life on the desk.
I still ask, if the parents knew this boy was disturbed, why did they allow him to have a realistic looking pellet gun? If the brother knew something was up, why didn't he talk to him or involve the parents, before school?
Don't blame the Cops for acting properly to protect others.
Is this the 15 year old in 8th grade?