Head teachers are being warned they should take firm action against pupils who stage anti-war protests or leave school to take part in demonstrations.
David Hart, the leader of the National Association of Head Teachers (NAHT), says heads need to stop children "running amok in the country's high streets" - or face discipline problems in the future.
The Metropolitan Police Commissioner Sir John Stevens is also calling on parents and teachers to get children away from the demonstrations and back into school.
Police estimated that about half of the estimated 3,000 protestors in Westminster yesterday were school-age children.
Sir John said officers were "disappointed" so many children were playing truant.
"We would urge parents to work in partnership with teachers and the police in dissuading young people from carrying out any further similar unorganised protests," he said
Some pupils are being suspended or even expelled after taking part in anti-war demonstrations, but some schools are taking a more lenient approach.
Head teachers' leader David Hart attacked that softer approach.
"Heads should ban all protests during school. They should take disciplinary action against any members of staff who encourage the demonstrations and against any pupils who are absent when they should be in school," he said.
They might benefit more from learning about the causes of war than by demonstrating against it
Mr Hart said children had plenty of free time in which to express their opinions and warned that schools which allowed children out to protest would in future find children wanted to leave school to demonstrate against other issues they felt strongly about.
"The right way to go about it is to give pupils the opportunity in school to debate the issues.
"They might benefit more from learning about the causes of war than by demonstrating against it."
And he said it was "deplorable" and "irresponsible" that parents were writing letters to schools giving permission for their children to be absent to go on protests.
Some schools are taking a tough line.
A number of students have been expelled from a South Devon college because of their actions during an anti-war demonstration.
The group from Torquay Community College are accused of spitting at police, verbally abusing staff and refusing to move from a zebra crossing.
In the south west of England, 20 pupils at Cape Cornwall School in St Just, near Penzance, were suspended after joining a march on Wednesday.
As feelings rose higher with the start of attacks against Iraq, some parents sent letters to schools saying they had given their children permission to go on demonstrations.
Pupils from Priory High School in Exeter, who joined a demonstration in the city centre said they had been given permission to take part by their parents.
Some schools have been anxious about the involvement of younger children in previous marches.
At Fortistmere School in north London, older children gave the school letters of permission from their parents.
The school had asked them not to encourage younger ones to join the protest because of safety fears.
Younger pupils staged their own demonstration in school.
The surge of feelings over the war has left head teachers with a dilemma: how to allow children to express their opinions while exercising their duty of care over them.
Schools have a responsibility for the health and safety of children and act as "in loco parentis" - parents to the children for the school day.
Teachers and head teachers appear split on the best way to handle the situation.
While the National Union of Teachers says children who walk out to join protests should be treated more leniently that general truants, the Secondary Heads Association, like the NAHT, says head teachers should not allow children to walk out and to treat those who leave school as general truants.
The other main teaching union, the NASUWT, also says children should not be allowed out of school to protest - whatever their age.
Up to 200 pupils at Helena Romanes School and Sixth Form Centre
in Dunmow, Essex, staged a peaceful protest outside the school gates on Thursday.
Headteacher Stephen Smith said there would be no suspensions:
"We recognise that this is a particularly difficult time," he said.
"We don't condone their actions but there's a degree of common sense which has to be applied."
At least 100 students at St Boniface School in Plymouth face being suspended after a protest on the Hoe and in the city centre.
They will be interviewed by the head teacher over the next few days before he decides what action to take.
Tom Healey, a spokesman at St Boniface, said: "We realise that some pupils have strongly held views about the present political situation.
"We have no problem whatsoever with them giving vent to those feelings.
"However, we have got a duty of care towards pupils, and those who play truant."
Devonport High School for Girls in Plymouth is taking a firm line, forbidding children from leaving lessons. It says pupils get the chance to air their opinions in citizenship lessons.
Girls staged their own anti-war demonstration on the school playing field on Wednesday.
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