At 14 you still have a way to go. So if by 'first AFCO meeting' you mean just popping in to say hi and pick up pamphlets etc you can get away with pretty much anything. If I recall, I was mooching around in jeans and t-shirt when I happened to pop in on the spur of the moment for the various branch brochures all those years ago.
But anything other than that, then I'd forget the casual look and go for at least smart or semi-smart. Remember the military - and especially the training enviornment - is more formal than the rest of society and what is considered acceptable on a daily basis in civvie street won't necessarily cut it. So if you are talking about interviews, then a suit would be the safe option. Failing that, then jacket and tie or if in the summer smart trousers, shoes and shirt / jacket combo without the tie - known as Planters Rig in the Officers' Messes would probably be fine. If you make it to OASC, it's a suit all the way.
Oh and it doesn't need to be an expensive suit, just make sure it isn't too shiny and modern - opt for style over fashion. M+S is a good bet for relatively inexpensive and fairly traditional suits. Leave the ill-fitting shiny ones to the mobile phone salesmen.
PS - don't choose a degree on the basis of what you think the RAF would like you to do, choose one you will enjoy doing and think you will be successful in. That way, you will actually enjoy your time at university and get the most out of it. Unless you want to be an Engineering Officer (or Lawyer / Medic), the RAF couldn't give a monkey's what subject you study as long as you pass. As I said to my interviewer when he asked why I hadn't gone to Oxford or Cambridge with my grades, I told him I thought university was about developing as an individual as well as getting a degree and I didn't think I would do much developing sitting in a library for 3 years. He seemed fairly happy with that as a reason.