It certainly provides a deterrent.
Why do these people pick 'soft targets' like schools? Because there will be little to no resistance. They aren't shooting up court-houses or police stations. Because they know they will be immediately be met with armed resistance.
If you make schools no longer the softest targets around - yes, you're right - they may just move on to another target, or simply add a box to their checklist "kill guard", etc. But at least it gives somewhat of a deterrent, and gives them more of a chance to fail at their sick goal.
It's the same reason we have armored trucks. How often do these guys get into shootouts? Almost never anymore. Certainly not in the past decade. Why? Why do robbers go for convenient stores instead of armored trucks? Those trucks carry a LOT of money. A liquour store might only have a few hundred bucks on a good night. That armored truck is very likely to have 5 figures at a MINIMUM.
Honestly ask yourself that, please.