I think part-time jobs can be fantastic for students, but it depends on the job and on how they get to school. If it's working in holidays, great, if they don't have a long commute by bus, great, so they can study in the week and do a day on the weekend, that works well.
If they are in a grammar, have to travel two hours a day, then no. If they are doing their school work on weekends to keep pace with four A levels (which many still are due to Further Maths, languages) then no. If they have a health condition which means they find studying plus a small amount of socialising exhausting, no.
I think some exposure to work, like doing temp work for an agency in the holidays, or one day a week Sunday, is good.
Beyond that it takes over their lives, I have a friend whose son worked weekends, weeknights, loved their job but has just got much poorer A level grades this year. Perhaps they would have done anyway, but they can't know that now.
It also depends on finances in the household, if you need the money, needs must, but if you can support them to make A levels the priority, and a part-time job to fit around that, I think that's better. No-one is impressed by a job as a washer up if they have been to uni, and they are better off getting strategic work or internships at that point.