One busload of kids would be horrific - but would not have the numbers needed to be a true Al Quida style terror attack. They would rather take out the student body of a large inner city school. The more minorities killed and injured the more hatred would be generated against law enforcement for disregarding the lives of minority children.
Not only would they have the hundreds of deaths needed, but they would also stir up a lot of hatred against our system.
10 school buses. Devastating