Schools in Scotland are holding lunchtime sex clinics for pupils as young as 13.
Pro-family campaigners say the service makes the issue of underage sex worse rather than better.
Twelve schools in Lothian, Dumfries and Galloway are offering students tests for chlamydia and gonorrhoea during after-school and lunchtime clinics.
And a school in Fife is showing posters advertising a local sexual health clinic which pupils can visit during their lunch break.
Norman Wells, director of the Family Education Trust, criticised testing for sexually transmitted diseases at school.
He said: “Clinics that offer confidential sexual health services to children under the age of consent are part of the problem, not the solution.”
He added: “The problem is that parents are kept in the dark.”
It comes as figures showed that Lanarkshire saw a 40 per cent increase of syphilis cases in a year, with 15 to 17-year-olds a particular concern.
South Lanarkshire Council said they would be working with the NHS to deliver a consistent message about sexual health in secondary schools over the next few months.
A Scottish Government spokesman said: “It is essential that young people have access to services and information when they require it.
“NHS Boards and local authorities are expected to ensure young people’s sexual health drop-in services are available within, or near, every school in Scotland.”
In June, it was revealed that diagnoses of sexually transmitted infections (STIs) rose 5 per cent in England last year – leading to a warning that people are continuing to engage in risky sexual behaviour.
Chlamydia was the most commonly diagnosed STI, accounting for nearly half of the overall total. People under 25 had the highest STI rates.