More than 5,500 alleged sex crimes in UK schools were reported to police in the last three years, BBC figures show.
Freedom of Information requests sent to all UK forces showed there were nearly 4,000 alleged physical sexual assaults and more than 600 rapes.
At least a fifth of offences were carried out by children, so-called "peer-on-peer" abuse, but details about the rest of the assaults are not known.
The government said schools had a duty to report child welfare concerns.
All UK forces responded to a BBC FOI request asking how many sexual offences were recorded by the police at schools during the last three academic years, from 2012 to 2015. Police Scotland provided figures for 2011-2013.
In some cases the victims and suspects were both five years old.
Eighteen-year-old Shannon Rooney, from Stirlingshire, who has waived her right to anonymity, was attacked in a storeroom by a fellow pupil when she was 15.
"I said, 'no', and he pulled me in, shut the door, and started to sexually assault me.
"Parents believe that you are safe in school and that nothing bad is going to happen, but this changes your whole outlook as to what school is."
Shannon's attacker pleaded guilty, but initially received an absolute discharge. On appeal he was put on the sex offenders' register for a year and given a year's community service.
One boy, who did not want to be identified, was sexually assaulted in a classroom by three of his friends when he was 15.
"My abusers were the most popular boys in the school, they played on all the sports teams.
"The principal at the time tried to put it down to 'rugby locker-room banter' and didn't seem surprised at all."
Jon Brown, head of Sexual Abuse Programmes at the NSPCC, said: "These figures are very disturbing, especially as many victims are so young and the reported offences took place on school premises. Sadly, we are not surprised as previous NSPCC research has illustrated the scale of abuse committed by young people.
"We know that for some older children, accessing hardcore pornography is warping their view of what is acceptable behaviour. And the very young - those of primary school age or even younger - may be copying sexual activity they have witnessed."
The National Police Chiefs' Council lead for child protection, Chief Constable Simon Bailey, fears the problem may be even worse.
"I believe these figures are the tip of the iceberg.
"It is good news that more victims have the confidence to come forward and report abuse, although - while I cannot prove this - I believe more child abuse is taking place.
"That includes children being raped on school premises."
University of Bedfordshire academic Dr Carlene Firmin runs the MsUnderstood Partnership scheme to combat "peer-on-peer" abuse.
"Most agencies struggle to offer an appropriate response," she said.
"In cases I have reviewed, investigations can take in excess of a year, during which time schools are often left holding the risk, with students taking sides of different young people ahead of a trial in the absence of an agreed way to manage the process."
According to the latest Department for Education statistics in England, 60 children were permanently excluded for sexual misconduct in schools in 2013/14. There were no exclusions in Wales and Scotland. Northern Ireland was unable to provide figures.
The government said sex and relationships education is compulsory in maintained secondary schools and many primary schools also teach it in an age appropriate manner, but campaigners say it doesn't go far enough.
The Children's Commissioner for England, Anne Longfield, said schools and professionals working with children must be more attuned to abuse, particularly when it takes place on school grounds.
"I want Personal Social Health Economic Education with a relationships and sex component to be part of the national curriculum.
"Every child needs to understand what is inappropriate or illegal behaviour."
A government spokesman said the Department for Education had published a package of updated child protection advice and guidance in March this year, which includes statutory guidance for schools and colleges on safeguarding children.
He added that the government is also committed to consulting on mandatory reporting of child abuse, and has published guidance on teaching children about sexual consent from the age of 11.
"Our statutory guidance is crystal clear that anyone who has concerns about pupils' welfare should refer to local authorities or the police if a crime is committed, and all schools must act swiftly on allegations."