Teenager Alvin Alushi sometimes uses the internet to help with his homework, sometimes to send messages, but mostly he goes online to play games with friends. But not all of them are people he knows outside of cyberspace. "We were talking in a forum about what we look like," says Alvin, 14. "I was going to give some details, but now I think it is better not to."
What brought about this change of mind was a visit by Peter Yapp, a computer forensics expert, to Alvin's school, Pimlico, in central London.
Peter, who is more used to advising multinational corporations on computer security, is one of dozens of volunteers going into schools as part of a campaign to promote safe use of the internet.
"Because I work in the industry, I see the results of things that go wrong.
My company does a lot of work with police forces, helping to investigate paedophiles. I want to prevent children from getting into the kinds of problems we have seen in our work," he says.
Today he is giving an extended assembly to Pimlico's Year 10, his fifth and final session at the school, having already spoken to Years 7 to 9 and 11.
After an attention-grabbing online frog dissection, he highlights the danger of encountering paedophiles on the internet and the importance of not divulging personal details to people you have never met.
But it is not just about protecting children from paedophiles. He also covers the legal aspects of downloading music, the risk from computer viruses, cyber bullying and the pitfalls of online shopping.
"Children are spending so much time online that they are at greater exposure to these dangers, and it is important that they know where to go if they come across a problem," Peter says.
Most school internet systems are fairly secure, in his opinion, and issues are more likely to arise from children using computers at home. However, this does not mean teachers should not play an active role in encouraging safer internet use, according to Carol Scott-McHale, Pimlico's head of ICT.
"We have a duty of care and would be irresponsible if we used the internet but didn't tell pupils about the dangers. Along with parents, it is our duty to ensure they are safe online.
"We have robust firewalls but the internet is changing all the time and it is important to stay on top of it. Although there are a lot of good things about the internet, it does have a nasty side."
The Pimlico School sessions were organised by the International Information Systems Security Certification Consortium, or ISC2, in conjunction with the charity Childnet International. ISC2 aims to send more than 100 volunteers - all private-sector computer security experts - to 200 schools. It plans to expand if the pilot stage is successful.
Research by Childnet found that 49 per cent of children have given out personal information online and 31 per cent have received unwanted sexual comments via email, text or instant message. More than half - 57 per cent - have come into contact with online pornography and 79 per cent of children use the internet unsupervised at home.
John Colley, head of ISC2's European advisory board, says the idea for the campaign came from a desire among security professionals to put something back into the community - but there is also a self-interested motive.
"We are increasingly seeing cyber-attacks carried out from home-based computers, where the attacker takes over the computer and uses it to send out spam email," he says. "If people have a computer, they have a responsibility to make sure it is secure, not just for their own protection - it could also be causing havoc for other people.
"If we get them young and start teaching them good practice, then they will be more responsible citizens in the future."
Children must remember the stranger = danger rule and not give out personal details or agree to meet unsupervised with anyone they have contacted via the internet.
Consider using filtering software and agree ground rules about what services you are happy for children to use. Suggest strategies for dealing with any content they are not comfortable with - for example, turning off the computer screen.
Encourage children to keep their personal information private, to delete pop-ups and spam emails, and use a family email address when filling in online forms.