But older ones have given their schools letters of permission from their parents.
One sixth-former, Sinead, said: "When I woke up this morning, I was so incredibly nervous. I felt sick."
"Freedom for the Iraqi people must not be brought about by needless bloodshed."
In Carlisle, the police were called to a school after hundreds of pupils protested.
The 11-to-16 year olds marched into the centre of a village chanting anti-war slogans.
Many pupils are facing suspension over their protests but most schools aren't quite sure whether to treat kids who walk out to protest as truants or not.