The ad for the Black Friday sale at Players is above. You can also get good stuff online, but Players is the last tennis specialty shop in town, which is nice to have as an option for a ton of reasons. Feel free to read more about racquets and strings from this page.
Here are some gift ideas for high school players:
A new racquet. Players will let you buy one for under the tree, then bring it back and start a demo trial to find the right racquet, grip size and strings afterward.
A tennis bag
Racquet Tune-Up from Players: A re-string, re-grip and vibration dampener.
New Replacement grip, or set of overgrips (lots of colors available)
Visors or Hats
Headbands / Pony-Tail Holders
Tennis Ball Hopper filled with practice balls (Wilson Trinity as the recommended ball since they'll keep their bounce)
Ball-Saver (will keep cans of balls more fresh for 2nd or 3rd use)
Head Tape to protect the top of the racquet (Head brand seems to be the best)