At the risk of sounding like an indignant insult monkey from the left fringe of our political spectrum: How dare you?
If I have learned anything from years of shaping young minds from the middle school years (I have shepherded my own children through those year as well as some religious ed teaching), I have learned that they are quite impressionable. You plant a seed and it will grow. Plant weeds and they will grow too.
Your planners clearly wanted to pick some scenario that seemed so ridiculously false on the face of it that no one would believe it. Unfortunately, every time some truant parent kills or abuses a child the papers claim them to be a "homeschool" family. So the possibility of homeschool violence is realistic to some. Sorry to say, but on an individual basis it is as unlikely as in any other walk of life. And as a matter group violence, knowing the independent spirit of homeschoolers, I'd suggest it is nigh on to impossible. We do get together for field trips and such, but our kids come first and foremost.
If any group were to be targeted for inciting group violence, why not suggest union violence. There is a long record of that. Try this ridiculously insulting scenario: A school bus is crossing the picket line during a teachers' strike and a Molotov cocktail is thrown into a bus. Yes it is ridiculous. Yes it is insulting. Yes it is more likely than a bunch of homeschooling moms (and one dad) getting together, making a bomb, and planting it on a school bus. We're too busy teaching the evils of evolution and the merits of creationism (or so it seems). Except that the homeschoolers might actually be able to muster the knowledge and resources to make such a bomb, while the teachers' union would probably be arguing whether to use kerosene or gasoline in the cocktail; then they'd try to drink it. (Just kidding. They wouldn't try to drink it.)
If you wanted a group, why didn't you select a far out, totally unbelievable one: the Martians are tired of us sending our little tiny SUVs to their planet and came to attack, picking school children in Muskegon County as their first targets. May be too silly to take seriously. But no one is insulted. And you get to push a political agenda against SUVs.
Or how about the most common perpetrator of mass violence, the lone nut case with a beef and a gun (or bomb). Here's the scenario: a former county employee, fired from his job as a school janitor for being drunk and refusing to seek treatment, blaming the county and MAISD for his woes, planted a crude bomb on a school bus, etc.
No one real is insulted. No one real is singled out. No political agenda is advanced to the children. The dangers of alcohol abuse can be demonstrated.
Really, though! Homeschoolers getting organized enough to perpetrate a coordinated violent act? We have enough trouble getting together to go on a group field trip or the First Friday Mass!