In the past very few people went to high school. Read Diane Ravitch's book on the history of educational methods & philosophy (Ravitch is a conservative & was in the Education Dept. under Bush sr.) At the turn of the century, high school attendance was about 5% of the population. Around WW I about 30% of the population went to high school, and it wasn't until the Great Depression that near "universal" high school education came into play. Even so, only about 70% of the population went at that time.
In the past, superintendents had the right to excuse kids from school attendance. That's what they did with the troublemakers, LD kids, etc. They simply didn't re-enroll them for school. Problem solved.
IOW, if pretty much only the kids that want to, need to, and are *capable* of going to high school, and if schools can expel troublemakers, then the dropout rate is going to be very low.