I wouldn't rush to make a big thing out of it, until I notice something serious about it. Remember, she's 15 (right, Laura?) years old. Some girls are very thin at this age. I was this way, my sister still is very thin and it's not an eating disorder, and it wasn't in my case too. I was 22 yo and at 5'1" I was about 88 pounds. There are different reasons for this.

In our schools, we pass a mandatory test in the beginning of the year and at the end - weight, height, and other medical tests if needed. A doctor could tell more about it.

If you notice that she doesn't eat at the cafeteria (if you have one), or if she eats, but she goes to the bathroom immediately, or if she's taking laxatives or other pills (very often a person with problems will try to take them as no one notices), then talk to her, talk to her parents or a councelor at school. Keep an eye on her, anyway.