Teenagers are putting themselves at risk by inconsistent and incorrect use of condoms, a study says.
Teenagers are confused over sexually transmitted infections
Only about a third of those aged 16 to 18 used condoms regularly, the sexual health charity Brook found.
The study also showed some were not using condoms properly and were using them only as a form of contraception, rather than to prevent infections.
Researchers called for sex and relationships education to become a compulsory part of the curriculum.
Two thirds of the 1,300 teenagers surveyed said school was among their top three most important sources of information on sexually transmitted infections.
All secondary schools are required to include sex education as part of their teaching programme, but there are no rules over the content, leading to complaints that the lessons are too biological.
Brook chief executive Jan Barlow said Britain's sexual health problems - in the last eight years gonorrhoea and chlamydia have rocketed among teenagers - would not go away until the problem was tackled "head-on".
"This report should focus the minds of everyone who cares about the sexual health of young people today.
"Condoms are the only way for sexually active young people to protect themselves from STIs," she said.
"Although most young people do now use condoms at least some of the time, the majority are still putting themselves at risk by using them inconsistently and sometimes incorrectly."
The survey found huge gaps in teenagers' knowledge about STIs.
More than half thought chlamydia affected only women, while a similar number did not know that emergency contraception could be taken up to 72 hours after having sex.
A third thought STIs could be caught from a lavatory seat.
A spokesperson for Terrence Higgins Trust said: "This is extremely worrying, and helps explain the huge rises in STIs across the UK.
"It is clear that we need to do much more to support young people in avoiding sexual ill-health, and that has to start with better quality sex education.
"We need to discuss relationships as well as the mechanics of sex if we are to help young people protect themselves."
Campaign on way
A Department of Health spokeswoman said sexual health was a priority.
"We are working to prevent people getting sexual transmitted infections in the first place and will shortly be launching a major public information campaign.
"We are also keen to improve access to sexual health care services, offering people more convenient options for getting screening and testing, including further education colleges and private pharmacies on the high street."
But the Department for Education and Skills said there were no plans to alter government guidance to schools on sex education, saying it was up to head teachers to decide on the content.