In high school, my mom kept telling me to "look at a yoga book to learn some stretches."  The angle of telling a 16 year old to look at a yoga book was not influential in the least but the theory was sound. 

In clinic, I has so many hurt athletes come in that are incredible inflexible.  The research out there is inconclusive, some say good and others say bad.  Well, here is the biomechanics take on this.  If you body is incapable of stretching to move the way you want to, then you will bend and twist some other part to make the movement happen.  Placing the body in poor mechanical positions can lead to significant injury, therefore, a little stretching doesn't hurt. 

Professional athletes have trainers that follow them around and stretch them...there is a reason.  So, here is a sight with a few good stretching exercises that you can recommend to your young athlete.  I like this stretching article because it highlights the basic groups of muscles that seem to cause injuries when tight or improperly conditioned.

Just don't tell them that it is yoga...it doesn't work.  Trust me.