If an American became an exchange student to Australia, how woul schooling be effected since...?

(first I want to apologize if there are a lot of typos, I'm not used to my iPod :P)

I'm an American considering becommimg an exchange student my juinor year. While I'm not actually sure if it'll happen, or where I'll go if it does, but I was wondering, since Australia and the US have opposite seasons, and there vacation is in the middle of our school year and vice versa, wouldn't that really mess up my school years, or would it not really make a difference? Would I be able to start my junior year in Australia and graduate with my class on time senior year back home? I hope this question makes sense...