Head teachers in England are more likely to face problems with pupils bullying online and misusing social media than in any other developed country, an international study says.
A report from the OECD think tank reported the experiences of more than 250,000 teachers in 48 industrialised countries and regions.
It showed particular problems with cyber-bullying in England's schools.
"It's the dark side of the modern age," said the OECD's Andreas Schleicher.
The OECD's education director called for more regulation of social media, rather than leaving individual heads to try to cope.
The study, from the economics think tank the Teaching and Learning International Survey, looked at the working lives of teachers around the world, with England participating but not Scotland, Wales or Northern Ireland.
The survey, carried out every five years, indicated an increase in bullying in England's schools - driven by online bullying and harassment and problems caused by social media.
Of the heads in England surveyed:
- 14% faced problems each week caused by "hurtful" material posted about pupils, compared with an international average of 2%, with the United States having the next highest proportion - 10%
- 27% faced problems each week caused by pupils receiving "unwanted contact" online - in the form of cyber-bullying, compared with an international average of 3%, with Australia having the next highest proportion - 16%
"It's clearly about social media," said Mr Schleicher.
In France, mobile phones have been banned from school - and the OECD education expert said education systems had to find a way of dealing with the impact of social media and internet use on young people.
He warned of a lack of regulation in England, which left schools having to find their own response.
"I don't think it's something we can ignore and let individual schools sort out," he said.
Apart from the emotional harm of bullying, he said, the misuse of social media was "hindering learning" and needed to be addressed at a wider level.
The survey also indicated that England faced a significant problem with a shortage of teachers.
Head teachers in England were much more likely to report their biggest problem was a lack of qualified teachers.
Mr Schleicher said this was "way above" what was typical of other developed countries - and that England was "pretty much on its own" with the scale of worries about a lack of teachers.
Improving recruitment, he said, was not just about pay, but would depend on making teaching more "intellectually attractive", with enough time for professional development and research and to "create a job profile that is a true professional career".
The survey indicated teachers in England worked among the longest hours of any developed countries, with 50 hours per week.
But much of this seemed to be administration or other work outside the classroom, as in terms of teaching hours, England was below average.
James Zuccollo, of the Education Policy Institute, said these "stark findings" showed that "in spite of the government's efforts over the last few years, there has been no reduction in teachers' workload".
Mary Bousted, joint general secretary of the National Education Union, said the findings should be a "wake-up call".
"The government must end teachers' unsustainable workload by tackling the high-stakes school accountability system which is fuelling the long-hours culture and driving teachers out of the profession," said Ms Bousted.
Paul Whiteman, leader of the National Association of Head Teachers, said: "Teachers are graduates who have many career choices open to them.
"We have to treat them well and respect their need for a work-life balance, if we expect them to stay."
Education Secretary Damian Hinds said: "We know that too many teachers are having to work too many hours each week on unnecessary tasks, which is why I have taken on a battle to reduce teachers' workload so that they can focus on spending their time in the classroom doing what they do best - teaching."