
Condoms, pregnancy tests and morning-after pills are available at hundreds of schools across England.
A survey by the Sex Education Forum found 29 per cent of schools in local authorities which responded had on-site sex clinics.
One in six offer the morning-after pill or tests for diseases like chlamydia, while all those with clinics provided condoms or pregnancy tests.
The results have angered some parents' campaigners, who said they were being left "out of the loop" by schools because they were not informed about the treatments given.
But Lucy Emmerson, a senior development officer at the forum, said the majority of schools would have consulted parents or governors before setting up clinics.
Ms Emmerson said it would be illegal for a school to breach a patient's right to anonymity.
She said: "Parents with children in those schools will know that the support services will involve sexual health advice and what the range of services on offer are.
"Also, health professionals always encourage the young person to talk to their parents about any problems they are experiencing."
Andy Hibberd, co-founder of the Parent Organisation support group, said: "It is not a problem that children are getting sex advice in school but the fact that parents are being intentionally cut out of the loop is wrong.
"If they want the morning-after pill, the school will sanction that and the parent will never know.
"We would say that this is the end of innocence."
The forum, an umbrella group which supports the development of better sex education in schools, sent questionnaires to local authorities across England and 70 per cent responded.
The young people's sexual health advice charity, Brook, is a member of the forum.
Brook chief executive Simon Blake described the survey results as "brilliant news".
"All the evidence shows that if good quality sex education starts early enough, it can be part of the solution to problems of teenage pregnancy and sexually transmitted disease," he said.
Last week, official figures showed the number of girls under 14 having abortions had risen 21 per cent between 2006 and 2007, from 135 to 163.
Among all girls aged over 16, there was a ten per cent rise in the same period, from 3,990 terminations to 4,376.
In February, Schools Minister Jim Knight launched a review of sex education aimed at reducing Britain's teenage pregnancy rate - the highest in Europe.
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