I think it is a great idea - but unworkable in some more rural areas, unless the school is going to lay on transport there and back - not everyone has a CAR. It would be fine for my DD, who lives a 5 min walk away from her school, and frankly, if she hadn't done her work, AND hadn't attended the after-school detention (how do the parents not know about the after-school detention, anyway? - if DD turned up at home when she was meant to have an after-school, I'd bloody well MARCH her back to the school!). The school e-mails the parents if their dc have got an after-school.
ONCE I have disagreed with the school about an after-school detention - At the time, I had NO computer access. DD booked the first available slot in the ICT suite - which was 2 days after the H/wk was due in. She asked for an extension, got refused the extention, and got the after-school straight away because the teacher "Didn't do lunchtime detentions". I refused to let DD go - she handed the work in as soon as her slot in the ICT suite came up.
For anything that involved DD's behaviour or not doing work through not being bothered, I would back the school.
However - If it was DS1, I cannot MAKE his father send him to a Saturday detention when it is his weekend, and I know Ex-H wouldn't send him - 1) To try to get ME into trouble, and 2) Because it would mean having to get up to get DS1 out of the house on time.
Also, if it was DS2 - His dad lives too far away for him to go to a Saturday detention when it is his weekend.
My solution to both would be to ask the school to postpone the Saturday detention by one week - so it would be on MY weekend, and I could strong-arm either DS1 or DS2 to the detention.
So, I still think, that in most areas, this is a GOOD IDEA. The threat would be enough - and if it wasn't, then realistically, no form of discipline would work with that dc, so a fixed-term exclusion would be the way to go for those that do not turn up for the Saturday detention. A fixed 'ladder' of punishments would let everyone know what to expect - and a Saturday detention is pretty far up the scale.
Break-time detention. Lunch time detention. After school detention. After school detention with HoY. Saturday detention. Fixed term exclusion. Permanent exclusion.
So a Saturday detention would be step 5 in the disciplinary process - you don't get that many steps in the disciplinary process when you are working before you get the sack.
I may even suggest this to my DD's school's HT - I am that sure that it would be a GOOD THING for discipline.