The Florida courts have come to the interesting conclusion that a BB gun is not a weapon and reversed the resulting school discipline imposed. This could happen to you.
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Initially, it is important to note that this is from the Florida state court (not federal) and is interpreting Florida law, so it has no direct application to Pennsylvania schools. Yet the lesson is a cautionary one for schools everywhere.
In that case, a weapon is defined very specifically as a firearm, any one of a number of other specific but inapplicable items, or the catch-all of a "deadly weapon." That later term is also specifically defined within Florida caselaw as "one likely to produce death or 'great' bodily injury."
Even after reading the case, I am not clear why they did not proceed with the BB gun as though it was a firearm -- it fires a projectile in the same fashion -- but that may just be another facet of Florida law. In any event, the question for the court was whether the BB gun was likely to produce death or great bodily injury. The ultimate decision was no.
Because the BB gun could not be defined as a weapon, the disciplinary actions taken by the school, which the school had taken predicated upon the student possessing a weapon, were reversed.
I suspect that the Pennsylvania courts would have found the BB gun here a weapon, but at the same time this would not have been an issue if the policy under which the student was disciplined had not been so narrow in what it prohibited. In the end, that is the cautionary message: figure out exactly what the school wishes to ban and ensure that the policy will accomplish that end.
The Florida case is A.H. v. State of Florida, 2011 WL 4809171 (Fla.App. 4 Dist.). (Sorry, cannot link to it).