They can either blow themselves up on the busses or walk into the school with the real students and do a Beslen. I hope my tinfoil is too tight, but they do like to attack innocents and children, and it would get the public's attention.
They wouldn't capture a school by riding school buses. But they could coordinate hostage scenarios in a town or across the country on buses.
Or do a Madrid or London series of coordinated bombings.
Any of a number of ways they could play it.
There were a number of dry runs before the 9-11 hijacking.
Anytime we laugh at ourselves for blowing up a lunch sack outside an office building, we'll never know for certain if they were just testing reaction time or maybe crying wolf to wear us out.