If you really, really, really want to graduate high school, wait till your free untill you try anything. I'm in high school now and while I do know of quite a few stoners who aren't caught, every so often there's a big drug bust, and a good clump of people get kicked out.
meh there isnt a 'good' age, I was 12 but then I was brought up with alot of people around me completely off their faces. I'm lucky I didnt do anything heavier than mushrooms and pot when I was younger, otherwise my head might have exploded. I'm 18 now and I can handle it, I'm old enough to set my boundaries and make up my own mind about it.
I think 15 is good for weed and salvia and i think 18 is good for mushrooms and acid. I have tried speed before but i didn;t like it and thought it wasn't worth it so personally I think you should do coke and stuff the addiction is so not worth it to even try the stuff.