Green card renewal questions?

I obtained a green card in 2008, (10-yr) then moved back to Korea, and I am still there. I heard you have to go back to the US every once in a while to renew your green card, but my parents refused to fund my plane trip, saying it was too expensive. So I was wondering if it expired/if I can get it renewed, I am aiming to go to the US for college, and it would be much, much harder if I was on international status.