I've been a small guy my whole life (my waist size has grown maybe one inch, if that, since grade 4). A couple years ago I spent a bunch of time eating a lot and working out to bulk up. I packed on 25 lbs in 4 months and worked my way UP to an XS (5' 9", 145 lbs), which I've been able to maintain. Before then, at 120 lbs, I didn't even try with clothes; it was pretty hopeless and frustrating. Even if I found something that fit, it didn't look good as it only highlighted the fact that I had no meat on my bones.
I would find something cheap that fits decent and you will probably still look better than most guys your age in suits. Topman sells suits in XXS (34" chest) that also tend to have a cropped length and shorter sleeves, which might suit your height. Not the best made stuff ever, but very wearable for a 17 year old (I'm 24 and wear their stuff).
At 17, it's pretty much a given that everyone is a work in progress. So don't go buying a bunch of suits if you expect to grow or aren't happy with your body. You're at the perfect age to start hitting the gym. I have no idea why I put it off for so long.