There are multiple reported negative consequences to working during high school. A study by Tulane University showed that working youths are less likely to complete four or more years of college. Ohio State University reports that substance abuse is higher for adolescent workers who work more than 20 hours, but that there are positive benefits to working less than 15 hours.
The Ohio State University study also notes that high school working has a positive effect on employment status and income, even a decade later. Working over 20 hours a week may allow students to become financially independent and so learn how to manage their finances and even save for college.
Although there are both negative and positive effects of working while in high school, it is likely that working too many hours will have some negative impact on school work. High-schoolers should probably not work more than 20 hours a week, with the ideal not over 15.