I'll be supporting my kids in getting some casual work...but will be pretty strict on how many hours they can work (ideally, 1 shift per week). However, it will depend on their sport as well as DS is heavily into Karate and will have his black belt when he is quite young and then will be teaching (assistant teaching) at least 2 classes per week...i do think this will teach him many "working" skills.
I was a macca's chick...and still have many friends from when i worked there. Also, it was good to have a "3rd place" so that if something went wrong at school, i had another source of friends.
I would also look at supporting him in getting some extra casual work over the school holidays rather than in term time...or encourage him to work in Year 8/9/10 but not for senior years.
Not sure...will decide case by case basis.