The Royal College of Nursing annual conference, in Harrogate, heard how some youngsters in parts of London are taking part in a practice known as "daisy chaining", where they get together in gangs to have sex.
Judy McRae, a sexual health specialist in the capital, 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. It is very new and is only just starting to be talked about."
Liz Allan, chairman of the School Nurses' Forum, said colleagues also reported that some children are known to sell sexual favours for money.
She told the conference about cases of children who feel 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. These are children who are being exploited sexually who prostitute themselves as a result of coercion, violence, trafficking," she said.
Steve Jamieson, an RCN sexual health adviser, highlighted one case involving a 14-year-old boy who was diagnosed with HIV contracted through sexual activity.
The boy was shocked to realise that someone so young could be diagnosed with HIV, he said.
Research involving more than 1,200 school nurses in the UK, conducted by the Royal College of Nursing found that 90 per cent of them were dealing with providing sex advice and support.
It also found that 65 per cent supported youngsters with substance abuse, while 90 per cent dealt with obesity problems.
The research discovered a school nurse, usually employed by the NHS, covered at least 10 schools, caring for an average of 2,400 pupils.This meant that fewer than 3,000 school nurses covered the entire UK.
The RCN has called on the next government to double the number of school nurses to meet growing need.
Labour has pledged to give every child access to a school nurse within four years. An RCN spokesman said it was highly unlikely this could be achieved.