The group left school at lunch and crossed the street to the southwest corner of Pleasant Hill and Deer Hill roads in Lafayette. For about 40 minutes, they stood on the vacant lot holding signs reading "Bring the troops home" and "Draft Bush."
"As non-voting citizens, the way to enact change is through protest and activism," said student Kennan Cronen.
The protest was not "just to be able to skip school," he added.
The gathering was peaceful until a student with a different point of view walked over, took a piece of paper from which an organizer had been reading aloud and ripped it.
"You're all hypocrites," he said before tearing up at least two signs and pushing a few students.
Three female students stepped between him and the male anti-war protesters to prevent the scuffle from escalating.
"This is a peaceful protest," student Allison Cellini told him.
Four Lafayette police officers came over and separated the students.
Plans for the walkout began about a week ago and organizers initially expected 200 to 300 students. Someone speculated that fewer people participated after Principal John Nickerson sent an e-mail Monday afternoon, saying students who leave school without permission could be disciplined and would not be allowed to make up missed school work or tests.
"While we do encourage student activism and engagement in the local and global socio-political world, we cannot condone acts that violate school policy and disrupt our instructional program," Nickerson wrote.
Two women from Crosses4Peace.org joined the protest. They are from a group in Napa that encourages people to put a white cross in their front yard or apartment window calling for an end to the Iraq war.
They were hanging banners on highway overpasses Tuesday morning when they drove by Acalanes High School and saw students protesting at the street corner. They joined the group with a sign of their own.