Well first, most people of course do not make the choice not to use private school; it's simply unaffordable.
At 15k a year, per child (plus all the flipping extras) it's become unobtainable for many, icluding the traditional middle classes.
That's why I would never judge anyone for not using private school (and I think the OP is wrong to). Most people, no matter what they claim on MN
don't have that kind of spare change.
However, the high cost does not mean it isn't valuable on a subjective level. It's only money after all
. You earn it, you spend it, you earn some more...
When we first chose our DC's prep, it was based on little more than gut feeling. The place felt right. There were acres of green space, used every day, which appealed and it was five minutes away.
Now my DC are big busting teenagers, I can look back and measure their prep experience more readily. Especially as I volunteered at the local primary for years, so I could see a good comparison.
These are some of the things I think were valuable to us...
- Large year group, split into smaller classes (4 x 15).
- A full time TA per class.
- Early flexible setting.
- Early introduction of meaningful MFL with native speakers.
- Sport every day. Competitive sport every week.
- Subject specialist teachers.
- Well resourced; science labs, ICT suite, music studios, art studios, gym, tennis courts yadda yadda.
- Music; orchestra, choirs, jazz bands etc
- Drama taken uber seriously with lots of public performances.
10. High academic standards (enough to secure my DS with a scholarship for one of the most selective secondary schools in the country). I perhaps didn't see this as clearly at the time, but boy was it good!
11. Extra curricular - every day, lots of choice. No need to do the middle class shuffle around town with tired hungry kids, sourcing tennis lessons and bell ringing camps after school

.
12. Looooooong holidays.
13. Like minded parents.
14. The school/parents partnership was very strong and
much was expected of us. Wouldn't suit a lot of people!
15. Homework.
16. Exams.
Now obviously, some people will think yuck homework, or whatever, but that doesn't make it less valuable to me...