I agree with you on certain levels, its a wonderful idea but you need to think it through as to are you being fair to the students themselves, if their having problem s see why, help.

Just because a student is making bad grades doesn't mean that their not trying, the class their in may be to advanced for them an don't understand the work. You could move them to a class that's right for them. When this is done you will almost always see an improvement in their grades. Some students are academic and some are athletic.

Another thing you can do is get them tutoring so they can get one on one help at a pace where they can understand exactly what to do on their class work in order to get better grades and play sports in the end. Math might be the greatest problem though because it is hard. Me and a couple friends took the same math test and we all ended up with a C average on our exam. One of my friends got a D and is being moved to a different class, I am going to miss her but its better because she will be in a class she understands, this is a perfect example of the situation I was referring to.

like i said, just because someone is making bad grades doesn't mean that their not trying, it may be the classes are to advanced for them or they may need tutoring to understand. As long as you are fair I don't mind having to have good grades to participate in sports.