The BMI is probably right but doesn't necessarily say anything about whether your weight is right or not; if you are only 13 you have less fat on your body than an adult woman and that's what BMI is about. I literally could not do the dead man's float at that age because I literally sank to the bottom of the pool, had no fat at all. I was not underweight though. The US Department of Health and Human Services says 18.7 is normal: Normal weight = 18.5-24.9 http://www.nhlbisupport.com/bmi/

Too, I see you are in England so the feet, inches, pounds versus metric or stone weight measurements might be wrong or confused/complicated?

Your test score said you have a 60 on eating disorders though (which again, you answered the questions so there is probably something there, the test isn't making up its own rules :-) so if you care enough to argue, there might be a problem there with weight and eating.