Well, in comprehensive state schools, if you're not part of the 'cool' crowd, you pick up symptoms such as depression, low confidence and low self-esteem. They really aren't ideal places for shy introverts - in fact they make people shy introverts. The teachers are too busy and stupid to notice the complexities of emotional bullying - and bullying is something that you can never solve, regardless of the action you take. Some kids are VERY clever at making your life absolute hell.
Private schools on the other hand 'promise' to make your child happy. Despite attending one of the best-reviewed state schools in my county, I still wish I went to a private school instead - not because I'd be academically advantaged, but because it probably would've saved me from years of bullying. My adolescence might've been bearable, and I'd probably be a happier person today.
Having said that, I'm thoroughly against private schools too. I think it's ridiculous to make a profitable enterprise out of education. I'd pay for my children to be educated in a place where they'd be happy, and not have to go through the same trauma and bull**** I went through, but I wouldn't want to give them an unfair advantage in academia. Life's more rewarding when you earn your own privileges...