Hmm, all depends on the sport, being active is said to increase grades for kids, but then again these are kids that are in regular classes, not honors or AP.
I am in AP classes, 2 or 3 a year, and I found that Football, Wrestling and sometimes basketball, can be detrimental to your grades, becuase of the intense rigorous activities that cause you to be so tired you need more sleep. My GPA went from straight A's to 2 or 3 A's and 3 or 4 A's but then again I am taking much harder classes than middle school, and I know I could do better, but I have definiately tried to go to sleep at 9 or something after a rough f/b practice and tired to wake up at 5 to finish hw but wouldnt finish it in time. So it can be done but is very, very hard to keep your energy up with sports like those, especially the football and wrestling. Plus the games/meets are very hard because those will take up your whole night.