Teenagers are taking part in a group sex activity known as "daisy-chaining", nurses have claimed.
School nurses say they deal with teenagers who have been sexually exploited
School nurses at the Royal College of Nursing's conference in Harrogate said colleagues in parts of London had become aware of the practice.
Nurses warn daisy-chaining, where acts are performed with multiple partners, puts teenagers' health at risk.
They said the problem showed how school nurses' work had changed from checking for "nits" or giving immunisations.
Concerns about binge drinking and drug abuse had also changed their role, school nurses said.
Judy McRae, a sexual health nurse in London, said: "Colleagues are coming across reports of groups of young people having sex in large groups.
"It is known as daisy-chaining and is obviously very worrying as far as sexually transmitted infections and pregnancy is concerned.
"As we understand it, it involves groups of older teenagers going round to each other's homes and having sex in a similar way as swinging. It is very new and is only just starting to be talked about."
Steve Jamieson, an RCN sexual health adviser, highlighted a case involving a 14-year-old boy who was diagnosed with HIV, which he had contracted through sexual activity, though not through daisy-chaining.
The boy had been shocked to realise that someone so young could be diagnosed with HIV.
Other sexual health nurses said peer pressure was causing young boys to become sexually active earlier.
Liz Allan, chairman of the RCN School Nurses Forum, said some children felt under pressure to provide sexual favours because they are part of a gang culture.
"Most school nurses at some time in their career will work with children and young people who are subjected to sexual exploitation, that's boys and girls who prostitute themselves.
"These are children who are being exploited sexually who prostitute themselves as a result of coercion, violence, trafficking," she said.
Ms Allan said children could also sell sexual favours for money.
Research involving more than 1,200 school nurses in the UK, conducted by the RCN, found that 90% were dealing with providing sex advice and support.
Two thirds said they had also supported youngsters with substance abuse, and 90% had dealt with obesity problems.
A spokesperson for Terrence Higgins Trust said: "Risky sexual practices like this are the fastest way to spread sexually transmitted infections like HIV.
"It's very worrying that young people are putting themselves at risk in this way.
"Sex and relationship education must improve so that young people know how to protect themselves and their sexual partners."
A spokesman for the pro-life charity Life said: "Children are emotionally and physically ill-equipped to deal with sexual relationships and are getting hurt.
"STI and abortion rates are soaring and the psychological damage to teenagers is immeasurable."