Year 12 student Sean Hinkley, 17, was sent home from St Peter's College last Monday when he turned up with the short dreadlock style he had worn last year.

"They told him the policy changed at the end of the year and dreadlocks were now considered an extreme hairstyle," his father Paul Hinkley said.

When Mr Hinkley contacted the school, he was told if Sean returned with dreadlocks he would be kept isolated in an interview room to study by himself and wouldn't be allowed to mix with other students during recess.

"When I asked what would happen if he went to a classroom I was told the teachers would be instructed to stop the class and no one would be taught," Mr Hinkley said.

School principal Tim Hogan said every family had been informed of the policy change in a newsletter.

"Students who are out of uniform cannot attend formal classes or any normal activities," Mr Hogan said.

"Students who choose not to comply need to make a choice whether they want to belong to the school community."

After spending a day in isolation and the rest of the week at home while the other year 12 students were enjoying a three-day camp, Sean said he was upset over the issue but had decided to cut his hair over the weekend.

"I just didn't expect it to cause this much of a problem," he said.

Mr Hinkley, who remains disappointed at the way the school handled the issue, backed his son's decision.

"You can't let a kid drop out of school so we have to conform to the rules. I don't think his hair is so bad."