The head teacher of a Kent girls' school has written a letter to parents raising concerns about pupils' use of the teenage networking website, Bebo.
Bebo is in competition with other sites like MySpace and Friendster
Tunbridge Wells Girls' Grammar banned the site at the end of last year but children can still log on at home.
Head teacher Linda Wybar said they had since become aware of girls posting images which could be "soft pornography in the hands of the wrong person".
Bebo UK said it had "always taken privacy and safety very seriously".
Bebo is a "social networking" website which allows young people to build personal pages, with profiles and pictures, and communicate with friends.
But concerns have been raised that material posted on the site could be exploited by potential abusers.
Mrs Wybar said: "We've just been monitoring it, going onto the site occasionally and keeping an eye on what was going on, what girls were doing and saying and the way they were presenting themselves.
"Some were putting on photographs which had them in perhaps quite strappy tops, reasonably short skirts and so on, and they could come across really as soft pornography in the hands of the wrong person.
"What an 11-year-old girl views as being perfectly innocent isn't necessarily what some more unsavoury characters would view as innocent."
Bebo UK has said there are "a number of features in place to help protect our users".
It is working with a Home Office task force and other similar web sites on implementing "good practice guidelines".
A new chief safety officer, Dr Rachel O'Connell, described as "one of Europe's foremost experts in internet safety", is being appointed by Bebo on 17 July.
The site displays prominent safety tips as well as links to user guides produced by wiredsafety.org.