Well, the reason you are getting more lucids in the weekends is because you sleep longer, therefore you have more REM periods.

However, I agree with Abra that a consistant sleep schedule does help, because I get up at six in the morning no matter what the day, and I usually have good recall. If you wanted to increase your amount of sleep on the weekends, you should get up early and do anything you need/want to do for two hours and then take a morning nap. (Only on the weekends, though, because you don't want to be late for school.)