File this under “I didn’t go to law school, but that won’t stop me from commenting on arcane legal matters”
It seems to me that a vehicle that is legal to drive on public roads, can’t be excluded from a school. If they want this to be a policy, how can they let, for instance, a parent with a Confederate Flag bumper sticker drop off their child, or park on school property for a conference?
As to clothing - if they want to enforce this ban, but refuse to enforce other bans then they seem to be on thin ice - “someone is offended” seems like a pretty low bar for reaching the “lets ban it” threshold.