I guess they proved how dangerous BB guns were after all...
Seriously, though, if I had to make the decision, I would allow the kids to bring in the ballistics gel, bring in whatever pictures, reports, etc. they needed, just leave the gun itself at home (maybe bring in a picture of the gun).
Although I would warn next year's class not to do science projects involving such dangerous materials (and cite the BB gun incident), as they can hurt themselves working on it.