Well, it would be a good start.
All they have to do is write to all parents living within say a 4 mile radius of the school and tell them they will be no longer welcome to drop children off in a car. The they will have to arrange transport, be it public or shared.
Or even, perish the thought, the kids could cycle in or be ferried in on the coolest bikes in the world:
This will unclog the streets, and will have the infectious effect of showing an upcoming generation the advantage of cycling.