Children are at greater risk of sexual assault from fellow pupils than abusive staff, a headteachers' leader has said.
There are more pupils than teachers on the sex offenders' register, said John Dunford, head of the Association of School and College Leaders.
Dr Dunford made the claim in the Times Educational Supplement.
He spoke out after Education Secretary Ruth Kelly announced a series of measures to stop adult sex offenders from teaching.
It follows revelations that a number had been cleared to work in schools.
Dr Dunford said the number of child sex offenders presents heads with difficult problems.
They have to decide whether to share information with other schools and how to teach youngsters "who can be sitting in the same class as their victims".
In July last year, Dwayne Best, 16, was detained for life after admitting raping a teacher when he was just 15 years old.
Other teachers have contacted the supplement's online message board with examples of young sex offenders in their classes.
One secondary school teacher said one of her pupils was placed on the register at the age of nine for raping his cousin.
The teacher said: "As he had been sexually abused himself and exposed to pornography at a very young age, it was possible to feel some pity for him - until he carried out three sexual assaults on Year Seven girls that year."
The controversy over sex offenders teaching in schools arose when it was revealed a teacher in Norfolk had been cleared for work despite being on the sex offenders' register for downloading child pornography.
Another man with a conviction for child abuse was then found to have been working as a supply teacher in Dorset.
On Thursday Ms Kelly revealed 88 known sex offenders had escaped a ban on working in schools since 1997, including 10 who had been approved by ministers even though they were on the list of offenders.
She said anyone convicted or cautioned for a child sex offence would now be banned from teaching and an independent panel will decide who can work in schools, rather than ministers.
Ms Kelly said she "deeply regretted" that news of sex offenders in schools had upset parents and vowed to overhaul the system.
But she warned there must be "no witch-hunts against hard-working teachers".