The evidence suggests that working during high school doesn't have much of a positive effect, and may have severe negative effects, especially as one gets closer to graduation.
In a longitudinal study of 4757 students in grades 8 to 12, researchers found that working was a negative predictor of grades, credits attained and occupation aspirations at all grade levels studied. Additionally, 12th graders who worked were found to have lower attendance, to be less involved in extracurricular activities and to be less likely to attend college . These effects were observed even when students worked few hours.
Another study found that students who worked less than 20 hours per week performed just as well as their peers academically and exhibited no more problem behavior than students who did not work. Working, however, did not positively benefit academic performance or problem behavior.
Although working in high school does not appear to be beneficial, students who work report improvement in metrics such as ability to get along with co-workers and supervisors, as well as increased knowledge of career interests.