No, they can't.That's very distasteful and disturbing. But there are no laws that say these type of billboards messages can't be put up.
I suppose Neal or the school could threaten to sue but that really wouldn't help defuse the situation.
It's someone(s) first amendment right to free speech (one could also claim petition also).
You are right, though. Nothing legally that can be done and not very tasteful but everyone has a right to speak their mind, tasteless or not.