Twenty pupils have been suspended from school for taking part in an anti-war protest.
About 250 pupils from schools and a college marched in Exeter
They were among a group who refused to join lessons at Cape Cornwall School in St Just, Cornwall, and instead led a march to nearby Penzance.
They eventually had to be escorted back to lessons by police.
Thousands of pupils across the UK have left schools in protest at the impending war on Iraq.
Many have already been told that if they do so they will be treated as truants.
Because they were unsupervised, unescorted and truanting from school, I rang their parents
Head teacher Robin Kneebone
Robin Kneebone, head teacher of Cape Cornwall School, said: "Because they were unsupervised, unescorted and truanting from school, I rang their parents.
"We always do if someone is truanting from school and they were asked to come and collect their children and they did so."
Meanwhile, about 200 pupils from Humphry Davy School and Penwith College staged an anti-war demonstration in Penzance town centre on Wednesday lunchtime.
Students in Plymouth and Exeter have also been protesting over war with Iraq.
John Bangs, head of education at the National Union of Teachers, said pupils missing lessons to attend protests should not be treated as seriously as persistent truants.
The union has written to schools urging them to encourage discussion of the issues surrounding the Iraq crisis in citizenship lessons, which Mr Bangs said might keep down the number of children going absent.
But Bob Carstairs, assistant general secretary of the Secondary Heads Assocation, said: "Our advice is not to allow this but to treat it as normal truancy and take appropriate action.
"If pupils wish to demonstrate they are perfectly free to do so outside of school hours."
On Tuesday, 200 pupils in north Somerset staged an impromptu walk-out.
Pupils, aged from 11 to 18-years-old, from St Katherine's School in Pill, near Bristol, left school for an hour before being persuaded to return to their classrooms.
On 3 March, thousands of children left their schools in Britain to coincide with a similar demonstration in the US called Books Not Bombs.
Hundreds of people gathered outside Parliament on Tuesday to lobby MPs voting in the debate on military action against Iraq.
Further anti-war protests are planned for the weekend.