The only time the speed of the projectile matters is when you are talking distance travelled.
The effective distance of your average pistol round is going to be a maximum of 100 yards and when counting in accuracy in a combat situation probably 50 yards (or substantially less with a shooter with little training).
A rifle bullet can be expected to be effective out to 400 yards or more.
The rifle has a big advantage when distance to the target becomes factor. The longer the distance to the target the more advantage the rifle has over the hand gun. The higher speed of the rifle bullet means it can travel a longer distance while still carrying sufficient kinetic energy to be effective.
In a school shooting situation, the distance to the target would be unlikely to be greater than 20 yards and in most cases would be under 10 yards.
So the relative speed of the bullet is unimportant to this debate.
The real question is; if the school shooter does not meet any armed resistance at all what is to stop him from killing students and teachers until he exhausts his supply of ammunition?
Excellent point.