Children as young as four have been excluded from school for 'inappropriate sexual behaviour', Ofsted has revealed.
More than one in five primary schools has raised the alarm over pupils' conduct.
A study of 69 schools showed that a total of 14 had witnessed this type of behaviour.
In eight school, pupils had been suspended or expelled as a result. Two schools reported a 'worrying lack of response from social workers'.
One head teacher was told that a child of five or six whose behaviour was displaying 'sexual elements' was 'too young for a referral and he might grow out of it'.
The Ofsted report urges teachers to keep details of every child expelled for this kind of problem and monitor any action taken by the relevant support services.
Mick Brookes, the National Association of Headteachers general secretary, said: 'Some of it could be innocent childish behaviour, but there are some quite worrying things going on.'
Ofsted investigated after figures revealed more than 4,000 children under the age of five were expelled last year.
Inspectors found that some primaries had suspended the same children more than ten times in a year.
Schools reported youngsters biting, persistent disobedience, swearing, running away from staff, kicking or hitting staff, throwing chairs and climbing over the school fence.
The report criticised one school for branding a child 'feral' in behaviour records and writing a behaviour policy with a 'negative tone'.