In most private schools the kids want to be there so the learning thing is what they want. They’d be crushed if for discipline reasons they’re sent home. In public schools it’s different. They don’t want to be there. They’d rather be at the mall with friends hanging out. It’s the learning environment.
At home and in school. What teacher wouldn't want to teach where students wanted to learn and were not just waiting for the bell?
That leaves the private and parochial schools a good base of teacher applicants who realize there is more to their job than money, and who are willing to sacrifice the public school pay difference in order to work in a more fulfilling workplace.
I went to both public and private schools as a kid. The private (parochial) schools saved me, academically, in just the last two years of High School. Education was definitely emphasized in my home as a child.
I went from race riots to a serene education-focused environment, and admittedly, managed to get an education in both environments, but the public school education wasn't in the three 'R's. Frankly, with the exception of a few other students who were determined to learn all they could (fellow 'curve busters') the students in the public schools were ripped off by the crap going on, and politics made sure that incompetent teachers were retained by the public schools at the expense of the students.
Needless to say, the lack of discipline in the public schools at the time was a major factor, and 'social issues' made it difficult, if not impossible to impose.