Stretch Catch

Your students will definitely use
this motion on court

Stretch Catch is included in USTA's QuickStart Tennis recreational coaches resource book for ages 9-10. It's simple and effective, and perfect for Tennis Homework. Since USTA is transitioning away from the QuickStart name, I don't know if these books are still available. To save you the trouble of hunting through their coaching resource page, here's the activity.

Player and coach (or other partner) face each other about five feet apart. Coach tosses ball underhand to player. Toss should be to forehand or backhand side, not directly to player. Player should lean to left or right to catch ball in air while maintaining good balance. Player should remain facing forward rather than turning to side to catch ball.

Progression
Too easy? Stand farther apart and/or toss ball farther from player.
Too hard? Let ball bounce before player has to catch it.