One case that could have significant impact over just how much power schools have to punish students for what they say off campus is Morse v. Frederick, which will go before the Supreme Court next month. Essentially, a student who was off campus at an event outside school hours displayed a banner that the prinicpal felt was inconsistent with the school's mission and subsequently suspended him for 10 days.
The Student Press Law Center has filed a brief with the court supporting protection of off-campus speech, which you can find here. While this case wouldn't seem to specifically apply to free speech in sanctioned school publications, the SPLC argues:
"(Such a) wide-sweeping view of school power over independent, off-campus student speech has the potential to chill all types of student expression."
Additionally, the SPLC goes on to say:
"It could be used to justify punishment of a student for attending a public rally against illegal immigration that school officials deem insensitive or for writing a letter to the editor of a community newspaper condemning gay marriage that the school decides is intolerant. A student distributing leaflets in the public park urging the legalization of marijuana for medical purposes could find himself subject to punishment as could a student who wears an athletic jersey with a beer company logo on a weekend trip to the supermarket with his parents."
A full press release on the briefing can be found here. It's a case that bears watching if you publish your own thoughts outside school hours ...