As a part-time strength and conditioning coach for a high school in San Francisco, I see a lot of teen athletes, who play lots of games yet do very little to keep their bodies/knees ready for them. This can be a recipe for disaster.
Girls are 8x more likely to suffer from ACL injuries as boys, so it is critical that girls (and boys too) do a series of exercises during the season as well as off season to keep their knees and core healthy and strong to minimize the chance of ACL injury.
My Top 5:
1. Basic Squat - Yes, it is "basic" but most young athletes can't do a good one. Feet at a little wider than shoulder width. Pivot from the hip back as though you were going to sit down. Try not to let your knees move forward. 3 sets of 10 reps.
2. Jump Squat - Load the body as though you were going to do a squat and jump. As you come down, absorb back into the squat again. 3 sets of 10 reps.
3. Lateral Bound (or 1 Leg Hop. See above pic) - Same technique as the squat, by keeping everything aligned, and you'll load up the single leg, and hop laterally as though you were jumping over a 1 foot picket fence. Land with a light deceleration and balance then repeat off landing foot. 3 sets of 10 reps.
4. Step ups - Can be performed on a bench or low stepper. Alternate 1 leg step and drive leg up to hip height. Lower slowly with same leg. Be sure to swing arms with the motion for balance. Be sure ankle, knee and hip are aligned. 3 sets of 10 reps.
5. Hamstring Curl on Physio Ball - 2 legs are elevated on ball. Create a glut bridge and then pull heels in toward the body, and extend back out with control. 3 sets of 10 reps.