The High School I went to had 4 or 5 permanent SRO's(School Resource Officers) for about 2000 students.
Even an equal number of active shooters would be hard pressed to take down 4 or 5 LEO's.
That being said the middle and elm school I went to were in more affluent areas. They do NOT have a single resource officer on hand. The local PD is about 19 officers total. I imagine you could get a LEO on site to either school within 2-3 mins of a 911 call but still.
Part of the problem is schools have been preparing for a disorganized attack by one or two high school aged children. The Amish School House and Now the Conn. shootings have shown us they need to worry about threats from outside as well.
My wife picks up 3 children at an elementary school for some working parents. Since Monday morning, there has been a marked police car sitting outside the school both days. I assume this will continue to be "SOP" indefinately.
Thats just a bad idea...after a couple of weeks the cops will become complacent, a lot like the cops I still see sitting in their squad cars outside movie theaters. It would be all too easy to just walk up and eliminate the officer. A roving, unpredictable patrol is a much better option. It might be a feel good option but I imagine most thinking people realize its just window dressing.