Just about every state has a Mandated Reporter law. Teachers, Medical Professionals, and yes, even Foster Parents are Mandated Reporters. One of our resident tile crawlers can probably explain better, but if we even suspect any type of abuse, neglect, or sexual exploitation, we are required by law to to report it to each state's version of Child and Family Services.
A therapist is a mandated reporter.
And yes, I wholeheartedly agree with Chris in having a diversion program because teenagers will be teenage knuckleheads. However, that should be a one time "Get out of jail free"/"Scared Straight" chance. Two strikes and you're out.