Two pupils were arrested and another taken to hospital after 150 pupils refused to attend lessons, protesting at new disciplinary measures at a Leicestershire secondary school.
Police were called to Bosworth college for 14 to 19-year-olds, in Desford, when the teenagers refused to go to lessons. Two girls were arrested for public disorder, but later released without charge.
A boy suffered head injuries after being hit by a bottle thrown during the protest and was taken to hospital.
The protest started after organisers handed out flyers at the college gates, chanting "strike, strike". The police persuaded the pupils to move inside the school's security fence, after a three-and-a-half-hour stand-off.
The anger was triggered by a disciplinary system introduced at the beginning of this term by Sue Rothwell, the principal. Under the system, pupils get a penalty point for bad behaviour, such as being late or talking in class. Parents are called in when the tally reaches 19, and possible suspension follows at 24 points. Positive points are awarded for good behaviour.
One 15-year-old labelled the penalty system as unfair, saying points were given for trivial matters and claimed that 70 students had already had their parents summoned to school: "They're giving points out all over the place for silly things," he said.
Previously the college's discipline had been praised by Ofsted. When inspectors last visited the college in March 2003, they said student behaviour was good.
Mrs Rothwell told parents in a letter last week that the new system was on trial until October half-term and would be reviewed then.
She said about 50 students had accrued at least 19 points since the system began and their parents had been informed. She said: "These meetings have been constructive. Many of these students have responded well and have not gained many further negative points."
However, it had not worked for all students. She said she had invited some of the students to a meeting after the protest to discuss their views and that parents would be informed of any revisions to the system in a newsletter at half term.
Tony Buttler, vice-principal, said: "We need to emphasise the positive points of the scheme and refine it by perhaps increasing the number of points needed before parents are called in. We may also review the number of points needed to trigger a detention."