the school is not offering lessons from 6am-8pm, sunshinenanny.
I was (in later years, admittedly, as that was when my mum needed the 'childcare' and was unable to collect me from school any earlier due to her job) at school from 7.30am until 9pm on some days.
I joined the boarders for breakfast, had time to finish any last minute homework/use the library etc before lessons started at 8.40 (well, form time, assembly etc).
after school (lessons finished at 4pm), I had loads of chill out time with my friends. we could:
swim, row, abseil, horseride, rockclimb, use the art/photography room facilities, use the library/computer rooms (if necessary for work) or just ramble around the grounds (extensive). there were dogs to walk (housemistress' pets), clubs to attend (in an umimaginable range of subjects), tv to watch - or just loafing to be done.
at my prep school, I was there from 8am until 6pm as a standard day - no option but to attend these times. again, lots of facilities and it was not constant lessons. at certain times of the year (school play times, major projects or scholarships to study for) I would be there later for a finite period of time.
as other posters have said, it is unlikely that any parents would use this to it's fullest extent - 6am-8pm 51 weeks a year. but it will be useful to many, at varying points.