Sex education in Wales suffers because school nurses are busy delivering vaccines, say experts.
Personal and social education (PSE) advisers warn that teenagers are missing out on specialist teaching during the current drive to immunise girls against cervical cancer.
The controversy over sex education was reignited last week as Westminster announced compulsory sex and relationship education (SRE) for five to 16-year-olds in England. It is already compulsory in secondary schools in Wales, but not in primaries.
Larger schools may have dedicated PSE teams, which include nurses, but teachers often tackle sex issues during mainstream lessons.
The number of full and part-time school nurses in Wales last year was 248, nine per cent less than 2002.
In August, the Assembly government reaffirmed plans to place a family nurse in every secondary school by 2011 to help with health-related issues such as drugs, obesity and sex education.
Sarah Morgan, teacher adviser in PSE, health and wellbeing for Newport city council, said the lack of staff qualified to teach SRE is a serious issue.
"School health nurses are highly skilled professionals who contribute to many schools' SRE programmes. But every time there is a vaccination programme, they have to make this a priority."
An Assembly government spokesperson said it is for schools to decide who teaches sex education and how it is taught.
Sue Allerston, PSE adviser for the Education and School Improvement Service, described form tutors teaching all aspects of SRE as "less than satisfactory".
She said: "The only thing that would move things forward is if there was dedicated money to train groups of teachers on a multi-agency basis."
Sexual health campaigners maintain that good quality sex education can lower teenage pregnancy rates.
But in its last annual report Estyn said only a minority of schools provided high quality sex and relationship education.
The Assembly government has published a new framework for PSE explaining what children should know between the ages of seven and 19. The guidance, which is not compulsory, builds on the foundation phase principles of health and emotional wellbeing.
PSE advisers said the guidance gives schools flexibility to teach individual children, who mature at different ages. Primary schools in Wales must also have a written sex education policy explaining what they will teach or the reasons for not providing sex education.
This has led to some confusion in schools about how and when to tackle difficult issues. Mrs Allerston said the most common query by primary teachers was about dealing with inappropriate touching.
Margaret Morrissey, spokesperson for the National Confederation of Parent Teacher Associations, said parents needed to wrestle back control by opting out of lessons.
"The dilemma is that when you're teaching sex education, those children that are taken out of the lesson will be filled in by others afterwards - and they probably won't get it quite right."
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