I don't have issue with the BB gun on campus, that is just stupid. I think the other charge is what is really the issue...
A second degree criminal damage charge can be brought if an individual intentionally damages someone else's property without their consent, and the damage is valued at more than $500, or if property of any value is recklessly or intentionally damaged by fire or explosive.
Sounds like they may have been shooting at other peoples property. I have heard of kids shooting out car windows with BB guns. So, I think the property damage component is the real issue.