(FORTUNE Magazine) – Working after school is one of the verities of American life, right up there with baseball and apple pie. But while there's nothing wrong with teaching Junior the value of a buck or Janie self-reliance, too much work can be, well, too much. Many studies show that, in general, the more kids work, the less time they have for homework, and the worse they perform in school. Says Laurence Steinberg, a psychology professor at Temple University: ''We've reached a stage where for many youngsters their main concern is not school but work.'' Steinberg, who conducted a two-year survey of 10th- and 11th-grade students from nine schools in Wisconsin and California, found that teenagers who toiled more than 20 hours a week cut class more often and were less involved in their & studies than those who worked moderate hours. Those who didn't work at all did best. One teen who overdid it is Ryan Smith, 17, a senior at Shenendhowa High School near Albany, New York. Between July and October last year, Ryan worked more than 25 hours a week at a Long John Silver's restaurant. He often got home at 11:30 P.M., found himself falling asleep in class, and rarely had time to do his homework. Ryan now works only ten or so hours a week at the local public library. His grades have improved, and he is looking forward to going to college.

Excessive work also prevents kids from getting a well-rounded education. Carol Harblin, another senior at Shenendhowa, found that a heavy schedule at McDonald's during her junior year kept her from pursuing extracurricular activities. Now she works only Saturdays at Wendy's and has time for ballet classes and field trips, including a jaunt to Washington for the Inauguration. Some critics object not just to how much kids work but to where they work -- overwhelmingly in fast-food restaurants. Says Joseph Kinney, executive director of the National Safe Workplace Institute: ''Fast food is the wrong kind of work. It's repetitious and mindless.'' Sheer elitism, counters Mark Gorman, senior director for government affairs at the National Restaurant Association: ''The argument is, 'If kids are not working, they'll be home studying. They'll go to Harvard and become computer engineers.' '' Still, it's hard to argue against reasonable restrictions on hours worked. In 1991, New York and Maine enacted laws that allow children under 16, for example, to work no more than 18 hours a week when school is in session. In July, Washington State will impose similar limits. Says Rich Berkowitz, a policy analyst with Washington State's House of Representatives: ''What are kids going to learn in 40 hours of work a week that they can't learn in 20?'' Some state industry groups are also joining the charge to convince parents and kids that extra hours on homework offer a better long-term payoff than more time behind the cash register. Since September, Texas Restaurant Association President Bill Daniels has logged 30,000 miles visiting members in his state. The message: ''Business must put education first.''