Stay as active and positive as you can and enjoy what you can of this lovely weather! (whilst not ignoring school work!)
A couple of little exercises on the serve today - kneeling down so not gonna burn too many calories but a couple of key aspects to developing a great serve. Pronation - the outward turning of forearm and wrist to present the racket face to the ball (assuming continental grip and you are approaching the ball with the edge of the racket), and internal rotation of the hitting shoulder/upper-arm which can contribute to up to 70% of your power on the serve within a shoulder over shoulder action.
Have fun with it