CHICAGO (Reuters) - U.S. youngsters aged 14 to 18 who work at retail and service jobs during the school year put in an average of 16 hours a week, often at jobs that are dangerous and unsupervised, a study said on Monday.

The report from the University of North Carolina said some of the working conditions found in interviews with a representative sample of 928 teenage workers violated federal law.

“Teens are exposed to multiple hazards, use dangerous equipment despite federal prohibitions and work long hours during the school week,” said the report published in the March issue of Pediatrics, the journal of the American Academy of Pediatrics.

“They also lack consistent training and adult supervision on the job,” it added.

The research done during 2003 found the teens worked an average of 16.2 hours per week during the school year, including nearly three times a week after 7 p.m. on school nights. Thirty-seven percent of those under 16 reported working after 7 p.m. on a school night, a probable violation of federal law, it said.

Despite federal regulations prohibiting teens under 18 from using certain types of dangerous equipment such as slicers, dough mixers, box crushers and paper balers, or serving or selling alcohol in places where it is consumed, more than half of the boys and 43 percent of the girls said they had done work that was prohibited, the study found.

Girls were more likely than boys to have jobs that involved handling cash -- exposing them to robbery risks, the authors said.

Boys reported less supervision than girls.

Working late “suggests the potential for interference with school or sleep, as well as potential for exposure to workplace violence that is more prevalent in the retail and service sectors than in other settings,” the study said.

“Many teens are performing tasks that are prohibited by current federal child labor laws. Our results also suggest gaps in both safety training and supervision of working teens because approximately one-third of the teens reported not receiving any safety training,” it added.

Among those who were trained, it said, some critical areas such as what to do in case of robbery or how to deal with arguments or fights among co-workers were not addressed.