Parents are the best example. Your teens will learn much more from your driving skills and behavior than they will from a professional instructor. Plus, they have probably been watching you closely for the past year or so, and have formed some of their attitudes from their observations.
Crashes are deadly for teens. In the United States alone, we lose 5,000 teens each year to fatal vehicle accidents. Some are related to lack of skills; others are related to substance abuse or distracted driving. Your involvement in giving your teen experience behind the wheel when you are in the passenger seat will greatly reduce the risk of your teen being in an accident.
More states require more hours practicing. The standards for teens to get a drivers license is higher today than it was when we were learning to drive. Many states now require at least 30 hours behind the wheel before issuing a license, and your teen's drivers ed program in school probably provides only 5-6. And veteran driving instructors suggest at least 40 hours practice. So your time with your teen will be most of the time he or she spends learning to drive.