1- Water-Always have a full water jug. Sports Drinks are good too. Also bring a cooler with ice. Can't really fit it in your bag but take it on the court also.
2- Racquets. Varsity players should have a minimum of two like racquets. During a match, it is illegal to leave the court to get another racquet.
3- Energy food- Fresh fruit or dried fruit, granola bars, nutra grain bars are good. Something to give you an extra boost when the match gets tough.
4- Two Towels, One to use to keep your hands dry and the other to wet and put on your head or neck. Sounds weird? Try it in the third set on a hot day.
5- Sweatbands or rosin bag
6- Cap or a visor. Professional or college logos are not permitted in HS tennis.
7- Extra socks. If you play more than one match a day, change your socks.
8- Flip flops- Put them on after a tough match.
9- Sweatshirt or hoody. The temperature can change quickly.
10- Pen and paper to make notes after match and also to start homework.
11- Extra overgrips and vibration dampeners
14- hair ties
15- tennis balls
16- Extra tennis shoes. What if your shoes rip? What if you leave your shoes at home?
17- a good attitude