Children as young as five are being reported to the police after being accused of sexual offences at school, new figures reveal.
Reports of sex crimes in classrooms has almost tripled over the past four years, from 719 in 2011-12 to nearly 2,000 in 2014-15.
In one case, a five-year-old girl was reported to the police for sexually assaulting a boy under 13, while another incident saw a five-year-old boy accused of sexual activity with a girl under 13.
Police in Wales were also called in to investigate claims a five-year-old boy had sexual relations with a girl of the same age.
The youngest alleged victim, also in Wales, was aged just four.
In total, 4,711 sexual offences at UK schools were reported over the past four years, with nearly a third of alleged perpetrators being pupils. Girls made up almost two-thirds of victims.
Alleged offences ranged from harassment to serious sexual assaults and rapes. Less than one in ten suspected cases resulted in criminal charges or summons.
Children's charity Plan International UK received the figures under the Freedom of Information Act, with 34 of 45 police forces across England, Wales and Northern Ireland providing data.
The charity, which described the findings as "alarming," said girls were especially vulnerable from sexual harassment and abuse in schools. It called for sex education to include sexting, issues over consent, and how to conduct healthy relationships.
Kerry Smith, the charity's head of its girls' rights campaign, said: "These alarming statistics show that we're failing young people when it comes to learning about healthy relationships and consent.
"Quality sex and relationships education helps young people to develop healthy attitudes towards sex and relationships while helping to tackle inappropriate and aggressive sexual behaviour."
One education campaign group, however, said the rise in numbers of reports could be explained by teachers becoming over-cautious to what could be innocent childish behaviour.
"Schools are becoming hyper-sensitive to any contact or comment even where, especially amongst young children, this is entirely innocent," Chris McGovern, of the Campaign for Real Education, told the Daily Mail.
"These days there is a greater awareness of sexual bullying and assaults. There is, also, a greater willingness to report them.
"The dramatic increase in the issue does, however, suggest that the problem is increasing and that it is symptomatic of a wider problem of poor discipline and control in many schools."