No school can ruin an otherwise good education.
900 hrs a year...six to eight hours a day, five days a week, given up to one's mandatory attendance at school, certainly can make a good education virtually impossible. It would take a very unusual individual to teach himself what was missing in an educational regimen consuming a third of every weekday; and determine what was false, and find the truth.
A dear friend of mine, having attended public school from k-12 with almost no gifted track academics despite a genius IQ, said it was like spending those 13 formative years working full-time in the sewers. Except people get paid for that, and have a choice.
If no school can ruin a good education, then no slave labor camp can ruin a good job.