PRACTICE PRACTICE PRACTICE!!!
You are on a highschool tennis team, you have been given an excellent opportunity to improve! You have 4 YEARS to become the best player you can possibly, all with the support of coaches and team mates. Give tennis your 110% effort and you will be a bad-@ss dude by the time you graduate high school! Power is not bad, but you should learn controll and technique first. Soon you will be able to keep all that power in the court by using topspin! Ask your coach for help. I am sure he would be thrilled to spend the needed time with you in order to improve your game.