Some girls can be contacted by up to 11 boys a night asking for nude images, the schools watchdog for England says.
In an Ofsted survey, girls explained that if they blocked boys on social media "they just create multiple accounts to harass you".
The report also found nine in 10 girls believed that sexist name-calling and being sent unwanted explicit photos or videos happened "a lot" or "sometimes" between their peers.
The watchdog is warning that sexual harassment has become "normalised" among school-age children. Students often do not see the point of reporting abuse and many teachers underestimate the scale of these problems, Ofsted says.
In the survey, girls said boys "just won't take no for an answer" when asking for explicit images. At one school, the girls told inspectors they can be contacted by up to 10 or 11 different boys a night asking for nude or semi-nude images.
Women's groups are calling for school staff to be trained to change the culture and for a government taskforce. Ministers say schools and colleges will be encouraged to dedicate training days to help staff deal with sexual abuse.
A BBC investigation has revealed at least 13,000 sex offences a year between under-18s have been reported to police in England and Wales between 2018 and 2020. Thirty police forces responded to a Freedom of Information request which revealed in about 2,000 cases, both the alleged victim and perpetrator were aged 10 or under. About 1,000 of the reported offences were about events happening on school premises. This echoes Ofsted's finding that inappropriate sexual behaviour is filtering down into primary schools.
'I was sent 50 or 60 images of privates'
Cerys, 21, said most girls she knows had received sexually explicit images from boys or men. She said: "When I was younger at school I received 50 or 60 on Instagram, Facebook Snapchat, images of privates that I didn't want to see.
I think the first port of call is to go to your mum...you feel like 'I don't want to see that, I don't know what to do with that and no one's asked for it'. It's not very nice to receive it." Lucy, 18, said that receiving nude photographs had become "so normalised" that if they appear in girls' social media message requests they "just delete it".
She said: "Some girls do get it very often but it's something you brush away because it's not something you think of that's out of the ordinary... and there's nothing you can do about it."
Lucy said she did not think teachers knew what to do about the problem. Cerys said: "We were told if you're going to be involved in this then the police are going to be involved. It was more of a scary assembly rather than let's get all the girls together and tell them how to emotionally deal with receiving these kinds of images."
Amy, not her real name, told BBC Radio 4's Today programme that when she was in sixth form she was raped after a party. Now 25, she believes there was a wider culture of sexual harassment that played a role in what happened to her. She said: "What was so challenging afterwards was when I tried to speak to friends I was fed back lines like 'Even if it was 80% him it was still 20% you' and 'Are you sure you didn't lead him on?'. She said a number of other girls told her they had had similar experiences and she knew she had to tell the school.
She said: "It wasn't just one boy and me. It was a number of girls being affected by multiple boys doing these kinds of things." Amy said she was able to speak to a teacher and the support was "overall overwhelmingly positive". But she believes the exchanging of nude images has "got much worse" in the years since she left school.
"Now I speak to younger people they just say [nude photographs] is what's expected. That's what being in a relationship is or that's what love is, or if you like someone you share a picture and if you don't then clearly you didn't like them," she said.
Ofsted visited 30 state and independent schools and two further education colleges and spoke to more than 900 young people about sexual harassment. Some 64% of girls thought that unwanted touching was experienced "a lot" or "sometimes" amongst their peers, while eight in 10 said the same of being pressured into sharing sexual images of themselves.
Children said sexual violence typically occurred in unsupervised spaces outside of school, like parties or parks. Pupils in several schools said harmful sexual behaviour happens at house parties, without adults present, and that alcohol and drugs are often involved.
Most students felt that the relationships, sex and health education (RSHE) they received at school did not give them the information and advice they needed to navigate the reality of their situations.
Girls in particular were frustrated that there was not clear teaching about what constitutes acceptable and unacceptable behaviour. Many teachers said they lacked knowledge on topics such as consent, healthy relationships and sharing of sexual images. Inspectors are urging school and college leaders to "develop a culture" where all types of sexual harassment are recognised and addressed. They also say time should be set aside within the RSHE curriculum for topics that youngsters find difficult, such as consent and the sharing of explicit images.
Ofsted's review came after thousands of testimonies about abuse were posted on a website - Everyone's Invited - and the government asked inspectors to assess safeguarding policies and experiences in schools and colleges.
In April, ministers also asked the NSPCC charity to run an abuse in education helpline, which will stay open until October. As of Monday, the helpline had received 426 calls and helpline staff have made 80 referrals to external agencies, including the police or social services.
Ofsted chief inspector Amanda Spielman said she was "shocked" that young people said it was a significant problem at every school the watchdog visited. "It wasn't in some, it was in all of them," she told BBC Radio 4's Today programme.
"This is a cultural issue - it's about attitudes and behaviours becoming normalised, and schools and colleges can't solve that by themselves." Education Secretary Gavin Williamson said Ofsted's review had "rightly highlighted where we can take specific and urgent action to address sexual abuse in education". But he said schools and colleges could not be expected to tackle these issues alone.
The Department for Education says teachers and school leaders will be better supported to recognise sexual harassment and abuse and teach confidently about issues of consent, online pornography and healthy relationships. They will be encouraged to dedicate staff training days on how to deal with sexual abuse and harassment among pupils and how to deliver the new compulsory RSHE curriculum.
The End Violence Against Women Coalition called for a taskforce made up of the government, education leaders, and experts on violence against women and girls "to advise on next steps and drive the rollout of a whole school approach". Peter Wanless, head of the NSPCC, said young people had exposed what was happening and their voices must shape a whole-school approach to preventing harmful sexual behaviour.
Geoff Barton, head of the Association of School and College Leaders, said: "It seems obvious that more must be done with greater urgency to tackle the misuse of social media and the availability of online pornography." A spokeswoman for Everyone's Invited called for an anonymous reporting system available in all schools. "This would help reduce the massive gap between incidents and reporting emphasised in the Ofsted report," she said.
The Report Abuse in Education helpline can be reached on 0800 136 663, on Monday to Friday 0800-2200, or 0900-1800 at weekends. It can also be contacted by email at