Nearly half of pupils are bullied every day at school, new research suggests.
More than two-thirds (70 per cent) of pupils had considered changing the way they look because of teasing, while a quarter (24 per cent) had felt suicidal because of bullying.
The figures, from a survey by the Diana Award charity, show that 45 per cent of pupils aged 9-17 are bullied at school every day.
The alarming data emerges as 10 million children return to school this week after the summer holidays.
Experts branded the findings "deeply worrying" and said many schools were "failing" their pupils by not doing enough to stamp out the problem.
Alex Holmes, head of the Diana Award anti-bullying campaign, said: "It is incredibly worrying for parents – when you send your child into schools you think it will be a safe and happy place.
"Young people spend 11,000 hours in full-time education – to think they can be abused is awful.
Mr Holmes added: "Our research shows that schools are failing to keep young people safe and happy, and that is unacceptable. For a child to feel suicidal because of the treatment they have had at school is totally unacceptable."
The charity – which was founded as a legacy to the late Princess of Wales and works in thousands of schools to tackle bullying – questioned 1,865 young people aged 9-17. Most of those questioned (72 per cent) were at secondary school and a quarter (25 per cent) attended sixth form or college.
Mr Holmes warned that bullying in the classroom was spilling into social media, with children being subjected to abuse on sites such as Facebook and Twitter.
He said that bullies, often emboldened by the anonymity the internet can offer, continued to pursue their victims on social networking websites – meaning the abuse did not stop when the school bell rang.
"Our research seems to suggest that the largest amount of bullying is verbal and face-to-face," he said. "But that is not to say that the drama in playgrounds doesn't turn to digital drama online.
"We are seeing bullying continue online. It amplifies the bullying – there is a bigger audience and in some cases anonymity."
The Department for Education said it was providing more than £7 million to support staff by offering training to tackle bullying.
A DfE spokesperson said: “Schools should have robust measures in place to tackle all forms of bullying. We have strengthened teachers’ powers to tackle bullying, by giving them the freedom to search for and delete inappropriate images from phones and other electronic devices. We have also made clear that teachers can discipline and investigate cases of bullying outside school.”