By Katherine Sellgren
BBC News education reporter
"You feel like a complete failure, you feel there's something you've done wrong, you feel inadequate as a parent and horrified because it goes against your own moral principles."
The parents of the bullies often feel they have "failed"
When she found out that her 13-year-old son had been bullying other children at school, Mary (not her real name) was shocked.
"It's been stressful, you do become the outcast or the leper."
And, having been bullied herself at school, the pain was all the greater.
"There's an element of doubt - you feel it can't be true, surely not," says Mary.
"Did I do everything I could? Should I have known? But I didn't know."
Her son was expelled from his fee-paying school in February for intimidation and for wrestling another boy.
But despite her revulsion at her son's behaviour, Mary is critical of the way the school has handled the situation.
"There was no warning - the school did its own investigation in a day and expelled him in a day," she says.
"We weren't party to the investigation and the decision was made without our consultation."
Mary believes her son needs help to "mend his ways" and says putting him face to face with his victims in a supervised meeting would help him to understand the consequences of his behaviour.
"It would help my son to appreciate what it must have been like for the victim and help him put himself in their shoes.
"The school should have come down on him really tough and made him address his behaviour."
This line is also taken by the charity Parentline Plus, which takes thousands of calls every year on the subject of bullying.
While many of these calls are from the parents of bullying victims, advisers also offer support to the parents of bullies.
Resorting to bullying can often be a sign of deeper problems
"It is essential that when schools and communities develop policies to cut down on bullying and to ensure community safety, the families of bullies are recognised as needing responsive and appropriate help with their family life and are not further isolated," the charity says.
"Without such targeted support, the lives of those bullied will continue to disentangle and the families of those doing the bullying will not be in any position to prevent this destruction."
Of course, for the victims of bullies - and their parents - the idea of help for the perpetrators might well hit a raw nerve.
Mary admits when she was bullied at school, she wanted revenge rather than help for her tormentors.
But she says such an approach does not get to the heart of the problem.
"It's natural and instinctive to say bullies should be punished - it's a normal human reaction, having experienced bullies myself.
"I'm not condoning bullying, I'm horrified my son was bullying, but it's usually indicative of something else going on.
"Usually the bully needs help too, because ultimately nobody wants to be doing these things," she says.