Like I said, I haven't read into that many cases about this subject other than the more famous ones. My only contention is that the off-campus events did not involve a fundamental right like the First Amendment. I'm not sure if you have more examples of where the school applied disciplinary actions against disruptive student speech that occurred off-campus.
To play devil's advocate a bit here, if we play on another clause in the First Amendment, would it be ok for a school to suspend a student for being Muslim during the height of 9/11 paranoia because it would cause disruption in the school? It seems the very principal in Tinker is a bit flawed in that student speech is so easily trumped by a standard like "disruption."
I was not too surprised at that ruling mainly because it felt like the Court was deciding based upon purely ideological lines. I maintain, though, that even in that case, the student may have been off-campus but was attending a school supervised event. So even if the decision was based on ideology, it does not entirely disregard the principals from Tinker. It's arguable that his sign could cause a disruption of the school's drug policies.
I only brought up Clarence Thomas as a comical notation. Nothing is quite as funny to me as a textualist that reads the First Amendment's clause that states, "Congress shall make no law . . . abridging the freedom of speech," and then goes on to explain that there should be no freedom of speech for minor students. Didn't realize the First Amendment actually reads, "Congress shall make no law . . . abridging the freedom of speech except for minor students who shall have no rights." Maybe that policy was hidden in a footnote somewhere in the BoR. As an aside, I am borderline against even Justice Holmes interpretation of the First Amendment from Schenck v. United States with his analogy of yelling fire to cause a false panic.