Oh, hun. You have an eating disorder and you probably didn't even know it. You're just barely underweight, which means that you're still in a fairly healthy range, but you certainly don't need to lose any more weight. Don't try to pressure yourself to have as low a weight as shorter girls because it will honestly never work. I'm 5'2", and at 100 lbs I'd have the same BMI as you. So don't get yourself down if you can't seem to get down to those double digits. You'll just make yourself very very sick.
I know it may seem hard, but try to get your calories up. You have a ways to go, but if you try to eat higher calorie foods, it will get easier after a while. Your body, at your weight/height/age and moderate activity leve, needs about 2200 calories. But don't try to get up to that number in a matter of days. Try to add foods you like that are higher in calories, like nuts and meat and dairy. Look at what you're eating daily and what you enjoy eating, and find high calories versions of these foods. Treat yourself to snacks you like if that will help you get to a healthy number. Your body is starved for calories and your metabolism is probably shot if you've been doing this for a while. So if you suddenly overload your calories, you'll gain weight. In fact, you might gain some weight anyway. But don't panic and starve yourself or exercise more. Your body can handle some weight gain, since you are underweight.
And honestly, if you're trying your hardest and you still can't seem to get into the 1000's or higher, then feel free to 'splurge' a bit and get yourself something unhealthy. Like fast food or ice cream, starbucks, candy. Anything that will add calories and tastes good to you. The way you're exercising, you can afford to eat a few unhealthy things...
Oh, and if the calorie amount I gave seems way too high, that's because of your age. As we age, our metabolism gets slower, so the younger you are, the higher your metabolism is. Plus, at 15, you're still growing, even if you haven't had another growth spurt.
If you need any more help or just want some advice, feel free to PM or email me (I can get on IM too, but email me to do so, since I'm never on anymore...) ED's are my specialty, I know more about them than I do anything else in psychology. So feel free to ask me any questions you might have. If I don't have an answer, I can always make something up!
(just kidding, I'll do research instead...)
Anyway, good luck and try to get your calories up! I know it's hard, but your body will thank you in the long run.