With the peer pressure at your age, it is understandable and normal. You need to understand 9 stones is only about 126 in pounds that's very close to Ideal wieght for you at 5'7". If anyone is telling you different, stop listening to them it is unhealthy. Love yourself how you are first, then if you still think you are big see a nutritionist or a doctor, but they will probably tell your not fat, also. I really think this one is just psycological. Your self confidence is the true issue to work on not your weight.
Also, did you know not eating causes your metabolism to lower, eating regularly and not skipping meals will keep that metabolism running strong, if you still think you need to lose a few.