I recommend Players Racquet Shop for all tennis-related gear.  You can also get good stuff online (tenniswarehouse.com, tennisexpress.com and midwestsports.com are tennis focused sites), but Players is the last tennis specialty shop in town.  Having that available is nice to have as an option for a ton of reasons (including the best stringers and stringing machines), so please shop local if you have the option.

Here are some gift ideas for high school players:

The Obvious:

A new racquet.  I believe 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

Tennis clothes

Less Obvious:

Racquet Tune-Up from Players: A re-string, re-grip and vibration dampener.

Re-stringing

New Replacement grip, or set of overgrips (lots of colors available)

Visors or Hats

Headbands / Pony-Tail Holders

Water Bottle

Tennis Ball Hopper filled with practice balls (Wilson Trinity as the recommended ball since they'll keep their bounce)

Sunglasses

Tennis-Specific Shoes

Ball-Saver (will keep cans of balls more fresh for 2nd or 3rd use)

Vibration Dampeners

Head Tape to protect the top of the racquet (Head brand seems to be the best)

Tennis Lessons