Because public schools in the 1950s were almost uniformly not unionised, and because their number one mission at the time was to attempt to educate, as opposed to brainwash, the students.
That's probably why. Not that all schools/teachers succeeded, but they still managed an 85-88% literacy rate.
50 years of gov't intrusion into public schools has indeed done the damage intended.
Actually, the damage began more than 100 years ago...