Children are often unaware of the risks from unprotected sex

The pilot project, in York, comes after known cases of chlamydia, which can lead to infertility in women, rose by 14% last year.

It follows concerns raised by the Commons health select committee that not enough is being done to encourage safe sex among young people.

Mike Proctor, director of nursing at York Hospitals NHS Trust, which is running the scheme, said: "We would stress that this screening would only be offered to those pupils who requested it."

The two York schools taking part in the project already offer emergency contraception and condoms to children aged 11 to 18.

Ignorance

Chlamydia is sometimes called the "silent infection", as it shows no symptoms in 70% of women and 50% of men.

If left untreated, it can cause infertility or babies to be born with pneumonia or conjunctivitis.

More than 81,000 cases were diagnosed in 2002 - an increase of 139% since 1996.

York Health Services NHS Trust is also asking nightclubs in the city to take part.

A Department of Health spokesman said: "We suppport the innovative approach being taken and will be interested to see an evaluation of the effectiveness of screening in non-traditional settings."

Earlier this year, Professor Paul Reiss of London University's Institute of Education told BBC News Online that pupils needed 10 times more sex education.

Ignorance of chlamydia could lead to higher rates of infertility, which would not be discovered until adulthood, he warned.

Official figures also show syphilis rates have risen fivefold in the past six years and gonorrhoea has more than doubled.