As someone who had to work from age 10 onward, I was curious about how many others had to do this – and the answer is, close to zero at that age!
Among high school students, the percent who worked has a wide variance depending on who did the survey.
This chart suggests relatively few students are doing paid work outside of school:

But this table, from a different paper, suggests around half of students might have been working at some point during the year – with most working part time, less than 10 hours per week.

The Role of Schools in Rebuilding
Less than half of the working students, in that paper, saved any of their earnings for future college expenses.
Many teens work during the summer – in fact, one paper says 3x more teen work during the summer than during the school year – thus, the majority of teens who held a job were working during the summer, not during the school year.
“However, holding a part-time job while still a full-time student may be detrimental to a student’s well being. Work could place extra stress on students and cause them to burn out and tire of employment and academics.
Studies show that intensive work habits are associated with higher dropout rates for white male sophomores and white female juniors (D’Amico 1984). Working too many hours per week has also been related to an increase in class-cutting, probably because employed students “use class-cutting as a strategy for coping with their excess role demands” (Barling, Rogers, and Kelloway 1995). Too many hours spent at work could also cut into a student’s chances to participate in extracurricular activities.
Working also cuts into study time for students. Studies show that students who are employed during their summer vacations do not experience a change in their grades during the school year, whereas students who are employed during the school year do. This suggests that having a part-time job during the school year instead of during the summer interferes with studying time for students (Oettinger 1999).
Holding a part-time job while still a student can affect grade point averages. D’Amico notes that “the
steady rise in high school employment coupled with the gradual erosion of academic excellence in recent decades may be more than coincidental” (D’Amico 1984). An increase in work hours significantly lowers a student’s GPA.Numerous studies show that the breaking point for too many hours of work is 20 hours per week. Students who work up to 20 hours per week do not have a significant decline in GPA; in fact, their GPA may even improve.
However, students who work over 20 hours a week have noticeable declines in GPA (Lillydahl 1990). This is probably because students who work more are more likely to not complete their homework and be absent from class. According to economist Gerald Oettinger, an increase in the number of hours worked per week with the number of weeks worked held constant is associated with a decline in grades. However, an increase in the number of weeks worked with the number of hours worked per week held constant is associated with a slight increase in GPA (Oettinger 1999). This suggests that a high frequency of hours worked per week crowds out study time, but a high frequency of weeks worked with a lower number of hours worked per week might actually help a student better manage their study time. Oettinger’s research also shows that with up to 20 hours of work per week, a student’s GPA only declines by an average of .05 grade points, which is equivalent to just one percentile. However, after more than 20 hours of work per week, the decline is much more significant (Oettinger 1999).”