hi,
i think your're going to have to accept the fact that you WILL put weight on, you cant stay that weight for ever and if you obsess over this you will make yourself ill,
you are only 15
you still have to grow...
when i was 15 i was probabily about 8 and a half stone, i was size 10-12, like you im 5 foot 5 and a half
when i was 21 i was still the same size (10-12) but weighed between 9 and a half and 10 stone,
you will put weight on naturally as you grow older just try not to worry about it,
i only ever got weighed in PE at school for some health programme we were doing at the time, i never even looked at the scales at your age, you shouldnt obsess over weight, you sound like you are slim, maybe a little underweight, tho i can sympathise, im a size 16 now! thats what having 2 kids does for you hehe but when i look back at pictures of me at your age i was soooo slim, but at the time i thought i was fat! but im older and wiser now all i needed was a bit of excercise, i mean i was a perfect size just needed to tone, if you think you look a bit podgey its not weight you need to lose you maybe just need to tone up a bit,
good luck anyway x