There have been a few national walk-outs in schools since this whole Iraq business started... You have to be careful in high school though, because they all have rules that allow student demonstration as long as it 'doesn't interrupt the learning process.' And, quite obviously, walking out interrupts the learning process, for you anyway. If you get enough people behind it then they can't suspend half of the students, but if it's only a few of you then be careful, because the administration will come down hard on your ass.
Some other, non-walkout ideas are to have a die-in (if done well, this hits hard)... at a certain time during the school day, have as many people as possible keel over and 'die'... hopefully wearing a sign saying something like 'how many civilians like me will die in this war?' Even if you don't get everyone to do it, imagine the effect of, say, 50-75 students strewn about the hallways and classrooms for 5 minutes.
A less controversial alternative, and one that they can't expell you for, is to have a 'stand-up' for peace at a certain time; get everyone to stand up from 12 to 12:15 or something like that, to show their support for peace. You're remaining in your classroom and not disturbing your or anyone else's educational process... the Supreme Court will stand up for you on this one (no pun intended).
KAMIKAZE: MAKING NS MORE CORRUPT ONE THREAD AT A TIME
*Proud member of Newschoolers for Peace*