I quit the first week of January because I'm job hunting. But I have done it for months at a time in the past for various reasons. It definitely lets you take a good look at your usage and decide if you're a user or abuser...a very fine line.
I do tend to stay up later when I'm smoking and therefore, get less sleep. If you just can't quit or don't want to, try only smoking on weekends. Just keep your grades up and keep the parents happy. After all, they pay the bills and have done a lot more for you than you may realize. (assuming your family life is reasonably stable)