Parents don't value my education...

I have always been a high achiever at school, and I'm currently in my second year of A-levels. I am the only person in my family to have continued education after secondary school.
The job I do alongside A-levels (which my family pressured me to get) has been causing me to fall behind drastically and underachieve a lot in my mock exams. I have to work there 22 hours per week over the weekend... which I'm pretty sure is illegal for a 17 year-old to do anyway.
Therefore, today I have decided to resign so that I can once again prioritise my studies.
However, my parents are not happy about this decision at all and are saying that I should be earning my own income like they did at my age. They care more about the fact that I'm leaving my job more than the fact that I've failed my mock exams!

What advice would you give to me, in terms of dealing with my parents' reactions? Is there anything I could say to them that would make them see that I have made the right decision?
I told my mum that a lot of my friends aren't working because they want to focus on their exams and getting into a good university, and she said that "isn't the way forward"...