First, consider the NEA's post-9/11 guideline for discussing terrorism with kids:

The National Education Association is suggesting to teachers that they be careful on the first anniversary of the September 11 attacks not to "suggest any group is responsible" for the terrorist hijackings that killed more than 3,000 people.

Now consider this story Michelle Malkin brings to light (emphasis mine):

Local school district transportation directors instigated the exercise because they wanted to test their abilities to respond to emergencies...

Students from Muskegon Community College and Reeths-Puffer will assist in applying makeup to add to the reality of the gruesome scene. Between 200 and 300 people will observe the exercise, including school bus drivers, school administrators, emergency personnel and evaluators from agencies across the state who will provide feedback...

The exercise will simulate an attack by a fictitious radical group called Wackos Against Schools and Education who believe everyone should be homeschooled. Under the scenario, a bomb is placed on the bus and is detonated while the bus is traveling on Durham, causing the bus to land on its side and fill with smoke.

Gosh, I guess the moms and dads who teach their kids outside of public schools must be a serious threat, given that we need to simulate a bloody attack from them.