Hey, I've got the exact same problem, 17 (got into bushcraft properly at 13 when I stumbled across ray mears "four seasons" episode on bbc 2, from then on I was hooked

), none of my friends are remotely interested in bushcraft and all mock me for liking it...dont care though

tbh, I dont personally think a course is that necessary...just do what i've done and watch every single ray mears and bear grylls episode ever made about 5 times each

haha. But in all seriousness they do help you to get a basic understanding of the key principles and then you can go on to decide where your most interested...e.g fire lighting techniques...general camp craft, DIY carving etc or tracking and tree ID...you just have to find what is best suited to your interests...for me I prefer general campcraft, shelter building and fire lighting, tried my hand at tracking and tree ID but can't seem to get the patience to learn everything...not with our exams going on at the same time :/ (are you doing A-levels?) also tried carving made a few spoons and started on a bow...so we shall see how that pans out.
As for doing it on your own, if you can find a nice patch of woodland nearby maybe get out and practice a few skills here and there...or go and camp out in them? surely your mates wouldn't mind going camping with you? just say you'll give them a few cans of booze or something that should tempt them

lol. After you've been out a few times with them and got the feel of things you could perhaps try staying out on your own for a night? I was 15 when I first stayed out on my own, scary at first but after you overcame that it was quite an enjoyable experience
anyway hope that helps
Jordan