It varies between private schools.There is a lot more that goes into that 99% than simply the quality of the education.
99% of my friends from HS went on to college ... but I'd say only about 50% from my graduating class did. Home life and the emphasis placed on education has as much to do with who goes to college as does anything else. Also, I'd say that if the parents are paying for private school K-12 that the kid doesn't have many worries about how they are going to pay for college either.
In all, I'd say that private education probably is better because every kid in there has parents that value education that highly.
In general, private schools have more flexibility and budget available to deal with disciplinary actions, student placement (No Child Left Behind) requirements, higher paid teachers (depending on location and prestige of institution), extra curricular activities, classroom aids, flexibility in teaching methods, etc.
I guess my point, given all of that, why would I not send my kids to Private school? Putting them in the best possible position to succeed is about rule #1 in life as a parent. Should I artificially inhibit their opportunity just because the system is unfair to people of lesser means? Make better life choices. Don't get the psych or art history degree. Get something that will translate into actual money and life opportunity. Money is the currency required for increased opportunity.
I don't have issue with people who decide to send their kids to public school. That's fine. It's just not what I wanted for my kids. And really, is sending my kids to private school any different than people choosing to move into better school districts when they do their family plan? I'll add, the public schools in my area are turrble. I however chose to live on the water so part of that decision was also knowing my kids would go to private school. I could have moved into the north part of the county where the public schools are phenomenal. Had we moved there, we would have sent them to public school.