I went to the University of Southern California for my undergrad. They take their football very seriously. I'm not sure how it is now that Pete Carroll is leaving and the glory days are over, but the football players were worked really hard. I don't know if any were academically successful - they basically went for "easy" majors and could be found in classes where the professor was sympathetic to athletes (those were also some of the easiest classes I've ever taken in my life). Yet they were still coming away with C's and D's.
I'm giving them the benefit of the doubt here, because I don't think they were stupid - from what I heard from them, they were worked really, really hard. I honestly don't know how or when they would study, because if they weren't in the gym or on the practice field they were probably sleeping. (On second thought, I heard that they were at a few parties, at least, so maybe there was some extra time.)
The basketball and volleyball players seemed to fare better, but I don't think they were worked nearly as hard as the footballers.
Which is to say, if you're going to USC or a place like it, don't do it. I don't imagine that you could excel in both classwork and football (according to their standards), and the second to worst thing that could happen would be for you to be mediocre in both. (Worst would be to be poor in both.) If you're going anywhere else, very carefully evaluate the training schedule that you'll be under and what's expected of you, as well as how the pre-med courses go. For us, some of the pre-med courses had tiered grading - only the top 15% would get an A (and that includes A-'s). If you're away doing football stuff you have a distinct disadvantage in those types of classes, simply because you don't have the time (and perhaps the energy/focus) that the average student would. Evaluate carefully. Being a football player might earn you some points on your application, but it won't outshine a mediocre or poor GPA, and it'll likely take away the time you'd have for other, more standard pre-med extracurriculars.