Auzzieboy, here's the thing. Don't worry about BMI tests, fat check or any of that. You're 15 years old, and there's nothing wrong with being 78Kg. If you eat right and are more or less active in someway or another everyday, you're fine. You're young and healthy, and should be happy about it. I know what being 15 is like.There's all the pressure to be "thin" and fit the "normal" weight ranges and "adverage" size, but you know what? If everyone was thing, normal and adverage, this world would be one shit-boring place to live in.
If you're worried about flabby arms and love handles (which EVERYONE has, by the way) do some sit ups and push ups once a day. Start at 10-20 of each and build it up as they start to get easer.
Love who you are, and don't try to be anything else. Break the mould.
. . . when the rich wage war . . . it's the poor who die . . .