The laws regarding these things, as well as the boards which certify and license physicians, vary by state, so it's difficult to make blanket statements.
Most states require that certain diseases, conditions, or injuries be reported to state officials. Typical injuries which set states off are suspected child abuse, gunshot wounds, extensive burns, workplace injuries, blah blah. Diseases might be cancers (in states which maintain cancer registries), STDs, or any communicable diseases. So if the physician detects a touch of VD, he may be required to report it to the state health authorities. I'm not aware of any state which requires that it be reported to the patient's mother.
Juveniles can be borderline cases but it seems that in medicine (and again, what's included in the practice of medicine varies by state - some include dentists, psychiatrists, nurses, etc; some don't) confidentiality is usually considered to extend to juveniles as well as to adults. Obviously there's some point at which confidentiality can't be a workable policy - care of four-year-olds, for instance.
In any case, the association with the physician must be voluntary. A required physical for, say, an insurance company, would generally not be confidential.