Children as young as five have been accused of carrying out sexual offences at school, new figures reveal.
And victims as young as four have been targeted, according to an investigation.
The number of sex crimes reported at UK schools has nearly trebled in four years from 719 in 2011-12 to nearly 2,000 in 2014-15, with rape, child prostitution and sexual grooming all recorded.
Children's charity Plan International UK, who got the figures using the Freedom of Information Act, said the findings are "alarming" and demanded the Government makes relationship education compulsory to tackle aggressive sexual behaviour.
Lucy Russell, the charity's UK girls' rights campaign manager, told the Press Association: "We are very concerned about these findings, sadly we are not really shocked because we have heard time and time again from girls in the UK that sexual harassment and sexual violence in schools is sadly quite commonplace.
"It is something they are really worried about."
In England, boys and girls as young as five were accused of carrying out sexual offences, while the youngest alleged victims were also five-years-old.
This included a five-year-old girl who was accused of sexual assault on a boy under 13, and a five year-old boy who was accused of sexual activity involving a girl under 13.
In Wales, the youngest alleged victim of a sexual offence was just four, although no details of the allegation or the child's gender were given.
Welsh police also investigated claims a five-year-old boy had sexual activity with a five-year-old girl.
The investigation also found the number of allegations of sexual crimes reported at schools has soared from 719 in 2011-12 to 1,955 in 2014-15 – an average of 10 each school day.
In total some 4,643 sexual offences were reported at UK schools during the four years.
Nearly a third (29%) of the alleged crimes over the four years are thought to be committed by pupils, while school staff - including teachers - are behind 15% of the allegations, according to the charity.
Girls and women overwhelmingly bear the brunt of this alarming trend, with nearly two thirds (66%) of alleged victims female, and 94% of the alleged offences committed by males, the figures showed.
Ms Russell said the girls are facing "day in day out, everyday harassment" in schools.
She added: "Sometimes it is passed off as banter or as just having a laugh and girls are expected to put up with it. But when we ask them how they feel about it, the answer is they feel harassed and bothered about it and that's a real concern.
"It's important to realise that the small things build up - every time you get away with something small it means you can try the next stage – so it is really important that schools give a clear message to boys and girls that unwanted sexual touching, sexual name calling isn't OK - it's harassment."
She also warned that very young children who exhibit sexual behaviour may themselves have been victims of abuse.
Ms Russell said: "The youngest cases are the minority so they don't represent the bigger picture, but absolutely if a child is acting out overly sexualised behaviour at a young age it should be a cause for concern to investigate much further.
"There are warning signs there that the child may have seen something that was significantly inappropriate for them, or they may have actually experienced abuse."
She said schools must take a zero-tolerance approach to sexual abuse and harassment.
The figures come from 34 out of the 45 police forces in England, Wales and Northern Ireland. Police Scotland declined the request on cost grounds.
The numbers relate to sexual assaults where the location was recorded as a school and may include some historic allegations as not all forces explicitly state if the offences are historic or not.
An NSPCC spokesman said: "It is deeply concerning that over the last few years thousands of young people have made allegations of sexual offences committed against them in school; a place that should be a sanctuary for children where they are safe from such horrors.
"The rise in reporting to police reflects a very worrying trend which should make all authorities and parents sit up and take notice and then redouble their efforts to keep children safe from sexual abuse, both inside and outside of school.
"The NSPCC are focused on addressing this problem and have introduced a number of initiatives - Pants, Speak Out Stay Safe - which are specifically designed to help children protect themselves from sexual abuse and to encourage them to seek help should they ever feel under threat."
A Department for Education spokesman said: "Schools are safe places and crime is very rare but any offence must be reported to the police. No young person should feel unsafe or suffer harassment in any circumstance.
"Sex and relationship education is already compulsory in all maintained secondary schools and many academies and free schools teach it as part of the curriculum.
"We are looking at all options to raise the quality of personal, social and health education (PSHE) teaching."