I disagree. All of this assumes a set plan, when it really only requires a desire to inflict harm without being noticed. If you can take the shot and then join normal traffic, odds are you're going to get away.
If a guy wants to hit a school, he could pick a school from the phone book and head on over. (Or, he could simply see the school from the road, and decide to hit it on the spur of the moment.) Pick a time where school traffic isn't heavy (as with today), case the joint for 15 minutes to find a decent spot, take the shot, and then make the quick right turn into traffic.
It'd be simple.