Secondary school pupils exchanging messages online with adults are easily duped into believing that they are speaking to peers by the use of textspeak and slang, new research has shown.

Teenagers were quick to assume that language use - including conversations about shopping, football and boyfriends - was enough to prove that online acquaintances were exactly who they said they were, even when pupils had completed internet safety training, academics found.

Previous research has revealed that 70 per cent of school-aged children and teenagers spend at least half an hour a day on the internet - and many much longer than that. Meanwhile, 31 per cent of pupils say they have been subjected to unprovoked sexual comments online.

In a new study, researchers from Lancaster University invited 785 secondary pupils to spend time chatting with strangers over the internet. At the end of each conversation, they were asked to guess whether the stranger was male or female, and a peer or an adult.

Only 10 per cent of guesses on average correctly identified the sex of the stranger. Among Year 7 pupils, only 6 per cent of guesses were right, rising to 17 per cent in Year 12.

When pupils were asked to say whether the stranger was an adult or a child, 16 per cent of guesses were correct. Among Year 7 pupils, only 10 per cent correctly identified the age of the stranger, rising to 22 per cent for Year 11 pupils.

The findings have prompted concerns that even young people who have been given lessons on digital safety could be vulnerable to online predators.

“Teachers expressed surprise at the amount of personal information the children disclosed, even though they had received internet training and were aware that they should not share such information,” the researchers write.

When asked why they had decided that their online acquaintance was male or female, pupils mentioned activities and interests that they felt were obviously gendered. “She said she looks at women in magazines and gets jealous of their figures,” one Year 13 pupil said.

“They were talking about going shopping with friends and trying on clothes,” a Year 10 pupil said.

“Because he plays football, and boys are more conscious about football than girls,” a Year 8 student said.

Another Year 8 pupil argued that the fact that an online stranger had mentioned “trouble with friends” marked “her” out as female. And a Year 12 student said that a stranger must be female, because she “just seemed to care” and “expressed concern over her friend’s boyfriend”.

For some, gender markers were even more subtle. A Year 12 student felt that phrases like “Aww, how sweet” were clear indicators of femininity, and a Year 10 pupil announced that “swear words and exuberant language” marked a stranger out as male.

Many students said that the use of text language and slang clearly identified strangers as contemporaries.

“I’m fairly certain that the person was about 14 or 15, because they.used a lot of slang and talked in a friendly manner right from the start,” a Year 12 pupil said.

Another student, this time in Year 11, said that “spelling errors.the amount of punctuation used.and grammar errors” marked the speaker out as a teenager.

Many made similar comments about the use of emoticons. “They used a lot of smileys, hahas and lols,” a Year 11 pupil commented.

The academics hope that their findings will allow teachers to have a better understanding of normal internet behaviour among students, and therefore to assess the safety of online activities more accurately. They also recommend that teachers and parents think about where pupils are able to access the internet - on computers, phones or tablets - and how visible they are to adults at the time.

Joe Hayman, chief executive of the Personal, Social and Health Education Association, said that many internet-safety lessons simply took the form of an abstract lecture about online dangers. Instead, he said, children should be asked to think what they might do if they received an inappropriate message.

“Children and young people will still take risks, and that’s an inevitable part of childhood,” he added. “But we can help them think through those risks, manage their feelings and make the right decisions for them.

“Going into that kind of depth requires a teacher who’s confident in what they are doing. We need more training, more support.”