Over 500 pupils marched to 10 Downing Street in London and held a sit-down protest, blocking the gates.
Several tried to climb the Downing Street gates, but police soon stopped that.
And in Birmingham, over 350 teens from schools in the city have marched to join anti-war demos in the centre.
They left their classes on Wednesday morning - but many didn't get permission from teachers.
The Head Teacher of Queensbridge School says any pupils who have walked out of school will be marked as a "truant".
The marches are all part of a massive Student Day of Action organised by Stop the War Coalition.
Demos have also been happening in Sheffield, Liverpool and Milton Keynes.
But it's a global protest: students and high school pupils in Sydney marched against war earlier on Wednesday and huge demos are planned across the US.