you should be fine.
i did my first GVBR when i was in year 9 when i was also 15 and the ride was from Mt. Hotham to Mornington via Gipsland (590km).
i actually went with my school on that ride, there was about 15 boys and 4 teachers. at the time i was still riding an XC bike with slick tyres, i was pretty skinny and didnt have the same kind of strength most of the the other guys had as they all played footy/basketball etc but i had been doing 50km rides along beach rd every weekend for maybe 2 years overcoming chronic asthma. despite all that i was still on par with the average fitness level amongst participants.
for training i just recommend mixing it up, if your worried about the ride devise a routine which is focused on building good stamina, get used to carrying lots of water with you too, i recommend a camelback.
will you be going with family, friends or your school?
i remember Christo mentioning in an old old old thread when he went with his highschool the best part was the opposite sex, every year i went the number of girls participating with their schools outnumbered us boys at least 3 or 4 to 1. there's nothing hotter when your 15 then chicks in lycra who dig bikes as much as you
so train up now so you can impress them with your hill climbing later hahaha good luck matey, some of the best memories in my life were on these rides.