At 119 pounds (which is what 8 1/2 stone is), you're normal weight if you're 4'10"" to 5'7" and underweight if you're any taller than that. See: http://www.nhlbisupport.com/bmi/
which is really the only thing you should care about when it comes to weight -- not what some boys say or some nasty girls say, but what is medically proper for your height.
I doubt that you're under 4'10" although it's possible
. 58 inches tall (which is what 4'10" is) is right in the middle of normal height for a girl of your age. Like contrex said, it's likely that you're still growing. 5'3 1/2" is about the middle of the range for women's height. Some countries have taller averages while others have shorter, but the average tends to be over 5 feet -- and at 119 you'd be at a healthy BMI.
So -- 1) ignore the kids who say you're fat. You probably aren't. But also 2) eat right and exercise. That will be more important as you get older than you may think now. It's not just about getting into a bikini, it's about being as healthy as possible for the majority of your life. And that's worth a lot more than how anyone looks in a bikini.