If that's so, then prohibiting beer ads, tobacco ads, political ads, ads for NAMBLA, ads for the local head shop, ad nauseatum, is also unconstitutional. What's being challenged here is the authority of school officials to decide what is or isn't appropriate to post on public school property.
Frankly, I think this is something of a straw man argument, because a more fundamental issue is whether or not the school should be selling ad space at all. If it's okay to do what the school is doing, then selling wall space for cigarette ads to raise money for lab equipment is no different.
Should public buildings be used as billboards? I think not. For one thing, issues like this one would be moot. For another, the idea of the word "PEPSI" painted down the length of the Washington Monument is somewhat distasteful to me, for some reason.
You're right, Imal. I believe that the courts say that if the school sells space to place your message at the school, you have the right to say what you want (you know, freedom of speech). Ads that go against previous limitations of free speech (you know, like statements advocating violence) would not be permitted. If the school is unhappy with what people say, then take away the policy of permitting people to buy space. And if someone puts up an ad for NAMBLA, you can bet that people will be picketing and using all lawful means (as per the 1st amendment) to make NAMBLA to take their ad down.