Originally Posted by
iamflyer
You may also want to consider that you could be risking your US residency status by remaining outside the US for a significant period of time. You should seek immigration advice before attending school in Canada.
Indeed, it's six months or more outside of US for Green Card to go poof.
I read somewhere that you are able to get an exception up to two years, but don't recall where I read that or what circumstances are.