Opinions: I have found that less serious mentees filter themselves out. The people who really want the mentorship are the ones that come back. As great as it is to mentor people, the ball is always in their court. I can't make somebody want to do something -- if they really want it, they'll ask for it, and they'll chase it down.

Those are the best students, anyway! Because things that are worth doing are often hard, and especially with programming, it takes lots of work to learn. You necessarily have to want it, and have to be taking it rather seriously. Then, the people who keep coming back for more are the ones who really want it.