Police officers were pelted with coins during an anti-war protest attended by about 4,000 schoolchildren in Birmingham.
Police estimate 4,000 people turned out for the march
Police officers were called to Wednesday's rally in Victoria Square following reports that some pupils had been throwing items - including a geometry compass.
A West Midlands police spokeswoman described the pupils as "buoyant rather than boisterous" and no arrests were made.
The crowds had dispersed by about 1400 GMT, with students moving on to Cannon Hill Park.
I wouldn't have taken part if I had known what would have happened
"We are investigating reports that cars have been damaged but we don't believe these incidents are malicious," the spokeswoman said.
The son of former junior health minister Lord Hunt of Kings Heath, Jacob Hunt Stewart, was among those students taking part in the protest.
Lord Hunt announced he was quitting his job on Tuesday, saying he had agonised over the Iraq policy for weeks.
Jacob, who attends Queensbridge School, in Moseley, told BBC's Midlands Today programme he was upset at the way the protest went.
"I was surprised at the way things went and I was deeply disappointed.
"A group of troublemakers did come along and infiltrated the peaceful protest that had been planned.
"I wouldn't have taken part if I had known what would have happened."
The anti-war protest is the second one to be held in Birmingham this month.
More than 300 children walked out of Queensbridge School on 5 March to take part in a rally.
Shropshire students are also holding an anti-war protest in The Square, in Shrewsbury.
Meanwhile, a peace activist from Redditch in Worcestershire has not eaten for a week as part of his protest.
Brian Haw has been holding a one-man demonstration outside the Houses of Parliament for nearly two years.
He says he will fast for as long as it takes to show his opposition.