Policy-makers may be wasting their time in trying to change harmful sexual behaviour through schools because teachers are the last people pupils want to approach on such matters, according to a study led by researchers from Aberdeen University.
Even the best curriculum is unlikely to influence young people's views and behaviour on sex, its report suggests.
Only weeks after ministers published their highly contentious sexual health strategy, the researchers found little impact on any key indicator from the "Healthy Respect" sex education project in Lothian over the past four years.
Despite this, Andy Kerr, Health Minister, announced last week a pound;1.9 million extension of the Healthy Respect programme from 10 secondary schools in the Lothian area to another nine secondaries, 13 primaries and a special school.
This would give particular priority to improved education on sex and relationships in schools, Mr Kerr said.
The team, commissioned and funded by the Scottish Executive, states:
"School-based staff, and teachers in particular, were the least favoured source of advice. Focus group interviews revealed concerns about lack of confidentiality, embarrassment and the possibility of being treated differently by a teacher after disclosure of personal information."
They say that the sexual health and relationship education programme (Share) that was introduced in 10 Lothian schools - one of four national demonstration projects - failed to turn around attitudes. Some young people are sexually active at an early age, their report states and starting sex education in S2 "may have been too late".
The researchers advise ministers: "Poor knowledge and behaviour outcomes from the Share schools must lead to questions of whether schools are the most appropriate venues for the delivery of interventions aimed at improving sexual health.
"A clear attraction for those wishing to influence young people's behaviour is the fact that education is a mass service delivery system - children move through in age cohorts regardless of emotional and physical maturity.
But in sexual health education, a proportion of any class must either hear the message too late or too early for them.
"It might be irresponsible to give no sex education at all, but from evidence it appears unrealistic to expect even the best curriculum to deliver significant changes in behaviour."
Schools are often reluctant to intervene in matters of sexual health and some believe that an "upstream approach" that raises pupils' expectations, achievements and aspirations is a better investment.
The Lothian study found particularly adverse sexual health statistics among young people in poorer communities. As a result, some teachers may favour the approach that lifts them out of a lifestyle characterised by low expectations rather than focusing on the close links between deprivation and poor sexual health, the researchers state.
They also maintain that "the issue of confidentiality versus child protection remains an unresolved but central dilemma for school-based interventions". Internet access in schools to sites on sexual information is invariably barred and "young people are aware of mixed messages about openness and sexual health issues".
The Healthy Respect project has so far focused on inter-agency training and collaborative working. School nurses, for example, contributed to drop-in centres for young people in Lothian. But the researchers found that young people had yet to feel the benefits of improved professional practice.
The Aberdeen team recommends that non-school staff should have much greater input to school programmes.
What the study found
* There was no increase in the low proportion of Lothian pupils who reported better communication with teachers on sexual health issues and Lothian.
* Around one in 4 pupils reported having had sexual intercourse by the age of 16 with no reduction after the intervention.
* There was more knowledge about condoms and how to obtain them but Lothian pupils were more likely to feel embarrassed about using them (especially girls) and more likely to think they would reduce sexual enjoyment (especially boys).
* Boys were reluctant to seek advice from drop-in centres and family planning clinics and girls were no more likely than boys to trust GPs.