A total of 200 school children staged a spontaneous anti-war demonstration near Bristol on Tuesday.
The protest lasted an hour
Pupils from St Katherine's School in Pill, North Somerset, walked out of lessons in protest at a possible war with Iraq.
The walkout was a surprise to staff, who said their main concern was the children's health and safety.
The pupils - aged from 11 to 18-years-old - said their message was that there should be no war and there must be another way.
One pupil said: "There should be a peaceful approach."
Andrea Arlidge, the headteacher, added: "As a school we are here to educate the children, and we hope that they are able to express their opinions.
"However, our concern today is with their health and safety."
Police attended and the protest lasted an hour before staff persuaded the students to go back to classes.
Teachers say they will be talking to students about finding "a more appropriate way" to express their beliefs.
North Somerset Council has said it will be issuing new guidelines to schools on how to deal with any future protests.