Since it was on school time, then the principal's in her rights to apply school rules. (Driving to school IS school time.)
I believe the principal, if her intent was based on rejection of the pro-drug message, is on decent footing.
No it isn't. If that's the case then he could have the school pay to get his car pulled out of the snow. I don't see that happening. If he was riding the school bus to school, then they might have a case.
"rejection of a pro drug message"
The really interesting factoid is that the message is neither pro or con for drugs or religion.
In court testimony it was entered that the sign is a meaningless collection of words with no message at all. It has no meaning and no meaning was intended. The banner is nonsense.