There are many topics you can talk about! It's not rocket science...you just pick topics that affect students and young teenagers. Here's a few...
1.) Studies- Talk about how hard it can be to concentrate In this society where we are always distracted.
2.) Socialising- Talk about the different hangouts young people enjoy and name a few and give ideas for those who don't currently have the opportunity or idea where to go and socialise.
3.) Beauty- For girls this will interest them. Talk about the latest fashion and how it's evolved. Talk about choice and how we have so much of it. Give your own opinion on how important it is to follow trends or whether you think trends are pointless? Or just give fashion tips.
4.) Film and Movies- Talk about the latest movies that will interest young people, give reviews on films that you've already watched and ask for recommendations. Make scenario's that if you were in that movie how would you feel? Or 'If you could be a movie character, which would you be and why?' etc, get them interactive.
5.) Celebrities and idols - Talk about the type of people and celebs that you admire and ask them who they admire, talk about how celebrities are influencing young minds and whether you think this is good or bad. Ask them their opinion. Or talk about the latest celeb gossip...
6.) Science- Talk about the wackiest inventions ever made and how useful they would be and how funny they are. Go into deep convo about scientists and their latest findings but make it interesting, something they can relate too.
7.) Global news - Talk about what's going on in the world now, and how It makes a difference and give your own opinion. Debate about certain political events and whether you think it's right or wrong?
8.) Morals and Philosophy- Talk about the right and wrong in this world, rule utilitarianism , religion, war, the philosophy of an idea and the mechanisms of what makes this world. Talk about general human behaviour and why we do the things we do.
I could go on! As there are so many things you can talk about...you just need to make sure that you write in a manner that will attract your young audience.