I used to be 10stone5pounds at 5ft8, 15 years old is this seen as average or overweight? 2hrs moderate exercise / week so it wasn't muscle weight...

Normal. Your weight calculates to 145 lbs. At 5 ft 8 in. This would be a bmi of 22, which is perfectly normal. If you did a lot of muscle building exercise, your weight would be higher as well as your bmi (muscle weighs more than fat). Some body builders have bmis in the overweight or obese category, but are not truly overweight.
You weight is. Relatively average. Continue your exercise, your weight may improve as you progress through and finish puberty.