Although you've enjoyed the time on your high school sports team, you may want to move on because you're too busy with academics or other activities. If your coach and your teammates depend on you, however, consider writing a formal letter of resignation to soften the blow and explain your reasons for leaving.
Give concrete reasons for quitting the team. Since the coach has put in a lot of effort to train you, he deserves a rational and intelligent explanation. If you find it difficult to come up with a solid list, perhaps you're reacting emotionally to something that happened recently and need to rethink your decision.
Thank your coach for her time and discuss the lessons and discipline she's taught you. Also, point out the ways you've benefited from the experience and by being a member of the team.
Explain how the team will benefit from your resignation. Talk about how you've been slacking off, getting distracted during games because you're preoccupied with other things or want to give a hard-working teammate a chance to play in your spot, for example.
Prepare to discuss your resignation in person with your coach. Consider the letter a way to completely express your reasons and feelings since you may become emotional and forgetful when you say goodbye in person.