ONE in five teenagers at schoolies celebrations this week will binge drink, take illicit drugs and have unprotected sex with little thought of the consequences, a leading child psychologist has said.

Michael Carr-Gregg said it was inevitable that some of the thousands of school leavers who arrived on the Gold Coast on Friday and early yesterday would run amok.

About 20 per cent of the estimated 35,000 revellers would experiment with an illicit drug - most likely ecstasy or ice - during the coming week, he said.

"Those who are recreational drug users would never consider going to a party and not popping something - that would just be unthinkable," he said.

Roughly the same percentage of schoolies would have unprotected sex without considering the potential risk of unplanned pregnancy or contracting a sexually transmitted infection (STI).

"They might be drunk or drug-affected adolescents who, in the moment, are not going to stop and think about condoms and STIs," Dr Carr-Gregg said. "Some young women will fall pregnant and some of those will go full-term and have a baby in nine months' time."

Research shows the majority of teenagers are sexually active and at least half of year 12 students have engaged in sexual intercourse, sexual health advocacy group Marie Stopes International said.

General manager Jill Michelson said infection rates were rising across the board, with people aged 15 to 24 especially at risk.

"There has been a huge increase in chlamydia notifications in the past several years and there is evidence that infection rates of other STIs, such as gonorrhoea and HIV, are rising," Ms Michelson said.

Almost all schoolies will drink alcohol at some point during the week but Dr Carr-Gregg believes only a minority will take it too far.