Children as young as five have been excluded from school for sexual misconduct, an investigation has found.
Hundreds of school pupils have been either permanently or temporarily kicked out of the classroom in the last four years after being involved in sexual acts, including watching pornography and sharing indecent images, according to figures obtained by the Press Association.
In an indication of a severe gender divide, data released by local authorities in England shows there were 18 incidents involving boys for every one incident involving a girl.
The vast majority of exclusions were on a fixed-term basis, with 14-year-olds the most likely to be involved in sexual misconduct.
There were 754 reported incidents between July 2013 and April 2017, according to the authorities who released figures under the Freedom of Information Act.
However, the true figure is likely to be much higher as the vast majority of councils contacted said they did not hold the information or refused to disclose it.
The data has prompted calls for sex education to be "dragged into the 21st century".
An NSPCC spokesman said: "Every child has the right to feel safe at school.
"Preventing harmful sexual behaviour through proper, up-to-date sex and relationships education is immeasurably better than excluding children after the harm has been done.
"By giving children the right information about sexuality, consent, risks and protection, we teach them how to make healthy relationship decisions, how to treat others and how to know when something is not right.
"Social media, sexting, online porn and dating apps did not exist when sex education was introduced on the curriculum a generation ago.
"It must be dragged into the 21st century, it must be consistent, and it must be offered in every school as part of a broader PSHE (personal, social, health and economic education) curriculum."
In March, the Government announced children would be taught about healthy relationships from the age of four, with sex education compulsory in all secondary schools, from September 2019.
Barnardo's chief executive Javed Khan said children who sexually harm themselves or others have often experienced some form of sexual abuse and trauma, and called for greater specialist support "to help them recover and understand why their behaviour is harmful".
He added: "It's vital all pupils are taught what a healthy relationship looks like so they understand what consent and respect mean."
Wednesday's figures show seven cases of children in the first year of school were involved in sexual misconduct during the four-year time period, although an age range breakdown was not available from every authority.
There were at least 40 incidents of children below 10 years old, the age of criminal responsibility, disciplined for misdemeanours.
Schools said children could be censured under the term "sexual misconduct" for a range of issues, including sexual abuse, assault, bullying, graffiti and harassment, as well as lewd behaviour.
Other examples included holding, distributing or requesting indecent images, accessing internet pornography, and sexual misconduct involving social media.
The figures do not include those where children were victims at the hands of staff or adult volunteers.
There was a surge in incidents as children reached secondary school - with 66 cases involving 12-year-olds, peaking at 120 for those aged 14.
The figures show the numbers trail off for those aged 15 (86 cases), 16 (41) and 17 (eight).
Sarah Green, co-director of the End Violence Against Women Coalition, said children have access to extreme images online without being given the guidance, protection or discussions about "respect, equality and acceptable behaviour".
She added: "These figures, which reveal really alarming behaviour in schools, show that girls and boys are being failed by those who should protect them and prevent this.
"The Department for Education (DfE), and school leaders and parents, need to take responsibility now for ensuring better child protection, better policies on bullying which recognise sexual bullying, and good relationships and sex education. We can change this."
The data was based on results from 15 local authorities with data.
A DfE spokesman said: "Sexual assault of any kind is an offence and must always be reported to the police.
"Schools should be safe places and we issue safeguarding guidance to protect pupils' welfare.
"As announced in March 2017, all primary schools will be required to teach relationships education and all secondary schools will have to teach relationships and sex education in the future.
"We want to help all schools deliver these lessons so that young people are equipped to have healthy relationships and treat each other with respect."
Figures released last month by the Government show there were 2,070 fixed-period exclusions for sexual misconduct in English schools for the academic year 2015/16, as well as 70 permanent exclusions for the same reason.
The data was collated from all state-funded primary and secondary schools, as well as special schools, but did not specify the age of children involved.