At King Edward VII comprehensive in Sheffield, more than 600 pupils were due to walk out as The TES went to press, while students at Forest Hill and Sydenham schools, in Lewisham, London, planned a school-gate picket line to prevent staff from entering.
Last Friday more than 200 Swansea pupils walked out to demonstrate outside the Welsh Labour party's annual conference.
International Socialist Resistance helped plan some demonstrations.
Many teachers said they would not prevent pupils walking out. Jane Nellist, National Union of Teachers' representative in Coventry, said: "If young people feel strongly enough about war to demonstrate, I don't think teachers should stand in their way."