Most schools are failing to meet students' needs in sex education lessons, casting doubt on classroom efforts to cut the number of teenage pregnancies and sexually transmitted diseases.
The Education Review Office assessed the quality of sexuality education programmes in Years 7 to 13 in 100 primary and secondary schools and found many were adopting a "one size fits all" approach.
Of special concern were the 20 per cent of schools that had "substantial weaknesses" in their teaching of sexuality education, the ERO found.
Two areas of particular weakness were discovered across schools: a lack of assessment of student learning and a failure to meet the needs of diverse groups of students.
The ERO has made a series of recommendations on how to improve sexuality education in schools, which its report described as "not effectively meeting students' learning needs" at most schools.
Sexuality education in schools became a requirement in 2001, but some students have been exempted since then, mostly on the grounds of religious and cultural beliefs.
The Family Planning Association said yesterday it had been concerned since 2001 that there was no consistency in delivering the curriculum.
The ERO report reinforced this view.
"Family Planning is not critical of schools," said chief executive Jackie Edmond.
"We understand the pressures they are under to develop and deliver a broad range of curriculum programmes.
"Our view is that schools would value and welcome clearer guidelines from the ministries about the sexuality education curriculum."
Research cited in the ERO report suggests that many New Zealand teenagers have incomplete or inaccurate knowledge about the practice of safe sex.
New Zealand has one of the highest rates of teenage pregnancies among comparable OECD countries, and the rate of sexually transmitted infections in youth is increasing.
So, too, is the rate of abortions.
New Zealand ranked second for teenage pregnancy rates in the OECD and had high rates of sexually transmitted infections among youth.
Last year the fertility rate for teenagers in New Zealand was 28.4 in every 1000 women aged 15 to 19. The United States was the only country with a higher teenage fertility rate - 45 pregnancies in 1000 women aged 15 to 19 years.
The ERO review found a strong focus on puberty in sexuality education at primary schools, while secondary schools focused more on contraception, relationships and sexually transmitted infections.
At the most ineffective schools, students identified gaps between their needs and the programme that was being delivered - particularly in relation to contraception, abortion, pregnancy, sexually transmitted diseases, non-heterosexual people and sexual intercourse.
ERO said it was concerned that many schools delivered the same sex education programmes from one year to another, in isolation from other aspects of health and physical education, and "with little consideration to students' individual needs".
This meant many of the teaching programmes were often repetitive, for example repeating pubertal change.
Assessment of student learning was highly effective in only five of the 100 schools that were looked at, and effective in 30 schools. Many schools used outside providers such as public health nurses, and did not have much of an assessment programme to find out how good they were.
In terms of meeting the needs of a diverse range of students, the ERO found that teachers in 34 schools were either effective or highly effective at meeting the needs of all their students.
The report noted the use of Maori staff in some schools and the inclusion of Pacific beliefs and perspectives in teaching at others.
Education Minister Steve Maharey said the report showed there had been "good overall improvements" since the last review of sexuality education in the 1990s, and "many schools are doing well".
But he also noted some schools were struggling and a "good practice report" would be valuable in helping them.
Women's Affairs Minister Lianne Dalziel said improvements in the knowledge, skills and attitudes of young people would result in more informed decisions about sexual and reproductive health and "will reduce risky behaviour".