There are few areas where I think common sense is more sorely lacking than in our public debates over free speech, and there's no better proof than two recent Supreme Court decisions.
But before we go there, let me state plainly where I'm coming from. First and foremost: The more overtly political the speech is, the more protected it must be. The First Amendment was not intended to protect pornographers, strippers or the subsidies of avant-garde artistes who think the state should help defray the costs of homoerotica and sacrilegious art. This isn't to say that "artistic" expression doesn't deserve some protection, but come on. Our free-speech rights were enshrined in the Constitution to guarantee private citizens - rich and poor alike - the right to criticize government without fear of retribution.
Now, there are commonsense exceptions to this principle. Not only can the state ban screaming "fire!" in a crowded movie theater, it can ban screaming "Vote for Cheney in '08!" in a theater, too (or, more properly, it can help theater owners enforce their bans on such behavior).
A better example of an exception would be schools. Students can't say whatever they want in school, whenever they want to say it, because schools are special institutions designed to create citizens out of the malleable clay of youth. Children aren't grown-ups, which is one of the reasons why we call them "children."
Making citizens requires a little benign tyranny, as any teacher (or parent) will tell you. If this weren't obvious, after-school detention would be treated like imprisonment and homework like involuntary servitude.
For a long time, we concluded the best way to protect political speech was to defend other forms of expression - commercial, artistic and just plain wacky - so as to make sure that our core right to political speech was kept safe. Like establishing outposts in hostile territory, we safeguarded the outer boundaries of acceptable expression to keep the more important home fire of political speech burning freely. That's why in the 1960s and 1970s, all sorts of stuff - pornography, strip clubs, etc, - was deregulated by the Supreme Court on the grounds that this was not legitimate "expression" of some sort.
Also, in 1969, the Supreme Court ruled in Tinker v. Des Moines, that students don't "shed their constitutional rights to freedom of speech or expression at the schoolhouse gate."