t’s a parent’s summertime nightmare. While you’re at work all day, your tween or teen is at home. Alone. With nothing to do. So what does she do? She turns your house into party central.This nightmare is exactly what happened to psychologist John Duffy’s clients, a family in a well-heeled Chicago suburb. “Every day when the parents went to work, their two teenagers invited all their friends over and had drinking parties,” Duffy says. “Some days there were as many as 100 kids at the house.” The couple didn’t learn about the parties until halfway through the summer, when another parent alerted them.For kids, there’s nothing more delicious than summer vacation: two-plus months free from school. But for parents of older kids, the summer months can be fraught with very real hazards: from drugs and alcohol to ill-advised risk-taking to auto accidents. (Most deadly car crashes involving teens age 13 to 19 occur between Memorial Day and Labor Day, according to the American Automobile Association).