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Girls at St Martins School aged 12 and 13 have been placing photographs of themselves in adult gear and wearing make-up, and supplying their contact details on public areas of social networking website Bebo.
"We were concerned some senior kids, especially girls, were putting themselves at risk in
a forum which anyone can access," said school principal, Bruce Bridges.
"A lot of undesirable people are trawling chat room sites and you read frequently of the grooming of young people.
"They (the girls) were modelling themselves as celebrities. Although I don't want to be alarmist, they were posing in a way we see every day in pap magazines.
"Five girls we dealt with individually and we alerted the parents. It was a mixed reaction in the main. Some expressed concern. They assured us they would look at it with the child but I suspect some parents took on board what we said with caution."
A parent alerted the school on October 12, after which it was found 15 pupils were on the site, many of whom were using Bebo sensibly in the private, not the public area, said Mr Bridges.
The girls causing concern were year 7 and 8 pupils. They had supplied their names, telephone numbers, postal addresses and school.
Lee Chisholm, operations manager for internet safety group Netsafe, said girls posting personal information was a concern since paedophiles are known to be accessing Bebo.
"We have seen photos of young males and females that are quite revealing and quite provocative," said Ms Chisholm.
"It is hard to say how often this happens. Bebo and others have millions of users and no-one is monitoring it."
Mr Bridges thought it was just the pupils' age and naivety that led them to put such personal material on Bebo.
"They are introduced into the chat sites and don't know the risks they are putting themselves into. I suspect it won't be the end"
Mr Bridges said the issue was being left to parents to solve but it was the school's "duty of care" to inform them of what was happening. He urged parents who allowed their children to use home computers unsupervised to monitor what their children were doing.
St Martins School blocked the Bebo site in the middle of last year and children are not allowed to use a computer unless a teacher is in the room.
"Our school server is checked regularly for inappropriate sites, which are blocked, and the person or persons who have accessed these sites are dealt with," said Mr Bridges. Fortunately this is very rare as our protection systems are very robust."
Last year the school dealt with cyber bullying and personal abuse on Bebo.