(FORTUNE Magazine) – If you think your teenager's after-school job builds character, instills leadership, and teaches him the value of a buck, think again. For middle- and upper-middle-class offspring, recent research suggests, long hours of labor during the school year can hurt grades and even get kids into trouble. About 42% of teenagers work -- mostly in fast-food and other service businesses that gobble up cheap labor. The National Child Labor Committee, a New York -- based nonprofit group, charges that restaurants and retailers sometimes violate federal child labor laws that prohibit 14- and 15-year-olds from working past 7 P.M. during the school year and ignore laws in many states that keep 16-year-olds from working past 10 P.M. High school students who work 15 or more hours a week tend to develop bad attitudes toward work and fall behind at school, say psychologists Ellen Greenberger and Laurence Steinberg in their new book, When Teenagers Work. The Massachusetts Department of Education reached a similar conclusion when it studied 11th-graders who worked 20 hours or more a week. They consistently scored lower than other teens on reading, science, and math tests. Greenberger and Steinberg, who interviewed some 500 teenage workers for their book, argue that the typical after-school job fails to teach new skills. Teenagers programmed to make coffee every eight minutes or flip a burger every 90 seconds learn to think of work as a boring, hateful task. Says Steinberg: ''The more kids work, the more cynical and jaded they become.'' Job stresses, according to the authors, can lead to drug abuse and heavy drinking -- financed in part by the teenager's paycheck. Some parents, though, cling to the Protestant work ethic. ''My son,'' one irate mother wrote the authors, ''learned more at Burger King than he ever did in high school.''