Have one in London Independent Day School (Boy). Pluses: three separate sciences, French (English Lang and Lit, Maths, RS, (then two to three of: German, Latin, Greek, Mandarin, Spanish, Italian, Russian, History, Geography, Art, History of Art, D&T, IT, Music, PE). Incredible art, music and science facilities and super sports facilities with: rugby, football, hockey, tennis, cricket, rowing, squash, golf, athletics, swimming, fencing, etc., all on offer. DS went at 9 and we have never had a moments worry, the communication is superb - have all the staff e-mails and have emailed perhaps five times in 8 years and only once for something fundamentally important, the staff are helpful and polite and on the one occasion we had a problem it was sorted effortlessly. The downside is 18K per annum plus expenses.
Have one in a small girls London comprehensive. Pluses: it's free and the staff by and large and the quality of the teaching has been OK. The communication is dreadful - inept and inadequate, behaviour management is poor, expectations are not high enough, the curriculum is narrower than we were led to believe, there is constant disruption. Front of house is sloppy, rude and unhelpful - a brilliant face for the school. Concerns are not dealt with and the tone is patronising. There is a total denial that a minority of children are bullies and intimidating and are a problem - I am the only parent who has complained about it according to the head. There is a growing group of Y7 and 8 mothers now sharing correspondence with each other that has been addressed to the Head. This is an oversubscribed school with super results and an excellent reputation. Fortunately our DD has only one more week to go and then she is free and off to the other sector from September.
Ultimately, it's about communication and one's confidence in the establishment. These may be two extreme examples but goodness we researched it carefully. Whilst I accept that hands are tied in the state sector in relation to exclusions etc., that is the real difference. The utterly dysfunctional and disruptive would simply not stay elsewhere.