Started at 15 @ DQ (had paper route, though from like 9-12 yrs old). Worked ever since with only a break in HS during sports season, in which I worked weekends. My senior year I decided to forego sports to work and pay for a car. When I went to trade school, I worked a 40+ hour work week and went to school. I'd literally change into my work shirt while driving to work. I got out of school at 3 and was supposed to be to work at 3. I was usually there by 3:10.
So, I see both sides. Yes, I did miss out on a lot of things. I wish I'd played football and gone out for track my senior year, but oh well. On the flipside, I believe those work experiences prepared me and molded my work ethic that I have now... If I had to give advice to someone in HS, I'd advise them not to work except in the summer.