Children must be taught more about sex and relationships as part of the school curriculum, according to a Government study.
A new report funded by the Department for Education called for a compulsory element of sex education focusing on the emotional side of relationships.
Pupils should be taught how to say "no" to their partners if they don't want to have sex, and how to deal with pressure from their classmates to engage in sexual activity, researchers said.
The evaluation of the Government's teenage pregnancy strategy was carried out by researchers from the London School of Hygiene and Tropical Medicine and University College London.
It found the strategy seemed to be working - rates of pregnancy among 15-to-17-year-olds fell from about 47 per 1,000 in 1998 to about 42 per 1,000 in 2003.
But the study warned the UK still had the highest teenage pregnancy rate in Europe.
Researcher Dr Judith Stephenson, from University College London, said some sex education was now compulsory in schools, but focuses too much simply on the biological aspects.
She said: "There should be more about peer pressure and more about negotiating relationships and resisting the pressure to have sex if you don't feel you want to have sex with your partner.
"Young people tell us that they want more on the emotional and relationships side and they feel it is already too biologically oriented."
A spokeswoman for the DfES said sex and relationships education was already compulsory.
But she added: "We trust teachers to use their professional judgement to decide which organisations can support teaching and learning in the classroom and which resources best support schools' sex and relationship programmes.
"We are taking action to improve the quality of personal, social and health education (PSHE).
"Since 2002, 2,500 teachers have received specialist training in PSHE.
"We have also asked the Qualifications & Curriculum Authority to develop guidance for PSHE."