Just to respond to Eugene's very thoughtful post below about Justice Alito's concurring opinion in Morse v. Frederick, I personally didn't find the line Alito drew to be particularly difficult or illogical. It may be wrong, but I don't think it's difficult or illogical. As Eugene notes, Alito wrote that he joined the majority opinion with the following understanding:

(a) it goes no further than to hold that a public school may restrict speech that a reasonable observer would interpret as advocating illegal drug use and (b) it provides no support for any restriction of speech that can plausibly be interpreted as commenting on any political or social issue, including speech on issues such as the wisdom of the war on drugs or of legalizing marijuana for medicinal use.

As I read this passage, Alito's view is that public schools can restrict speech so long as a) "a reasonable observer would interpret [the speech] as advocating illegal drug use" and b) the relevant speech cannot "plausibly be interpreted as commenting on any political or social issue, including speech on issues such as the wisdom of the war on drugs or of legalizing marijuana for medicinal use."

  Under this approach, there are a range of different statements that student speech can try to make, as well as a range of different interpretations that school administrators can make of the student speech based on the context. To determine whether the speech is protected, the court should first inquire if "a reasonable observer would interpret [the speech] as advocating illegal drug use"; if the answer is yes, the next question is whether the speech could be "plausibly . . . interpreted as commenting on any political or social issue, including speech on issues such as the wisdom of the war on drugs or of legalizing marijuana for medicinal use." If the second answer is "yes," then the speech is protected; if the second answer is "no," it is not. The result is a narrow type of student speech that school administrators can ban: in a nutshell, speech reasonably read as encouraging the use of illegal drugs that doesn't seem to have any other message.

  Eugene suggests that the line between the two categories isn't coherent, but I disagree. As Eugene notes, speech can both advocate illegal drug use and comment on a social or political topic. But I read Alito's part (b) to account for that and deem the speech protected (or at least to say that nothing in Morse permits it to be banned, which I assume means by default that it is protected). The speech is only unprotected if it advocates illegal drug use and can't reasonably be read as commenting about a political or social topic. Thus, student speech like "homosexuality is an abomination" would be protected because it makes a comment on a political topic. At least that's how I read the opinion.

  Eugene next suggests that urging someone to violate the law is implicitly speech about a political topic. If you urge someone to break the rules, you must implicitly be arguing that the rule is bad. That makes some sense in theory, but it's not how I recollect high school. Back in high school, student opposition to school rules was partly about rebellion for the sake of rebellion. If school administrators announced a rule, a subset of students wanted to break it just for the thrill of opposing authority. That's what press reports suggest this case was all about; unfurling the banner was "a prank [designed] to cause a stir" rather than speech designed to communicate a particular set of views about a political or social topic. It's not the only way to interpret the banner in this case, but I think it's a plausible interpretation.

  To be clear, I'm not sure I agree with the line Alito drew. I personally find the idea that pro-drug student speech poses special dangers to student physical safety to be pretty odd. What's the connection? Is the idea that students will hear pro-drug speech, start doing drugs, become addicts, and then become violent in class? Alito doesn't say; he points to the majority's argument, but that majority's analogous point seemed to be about the harms of drugs among schoolchildren generally rather than the threat of physical violence. And as Eugene rightly notes, we don't know what other categories might also pose these "special dangers." But while I'm not sure I agree with Alito's line, I think the line is likely to be a relatively coherent one in practice.