The high/low system was started to group kids based on age. For example: If you were ready to start kindergarten and had turned 5 by December 2nd, you could start kindergarten in September. If you turned 5 after December 2nd, you would start kindergarten in February. Back then, school did not start until right after Labor Day in early September. Now children start the school year in mid to late August. Back then, the spring semester began in early February. Now the spring semester starts right after New Year's Day.
When the high low system was in place, the kindergarten teachers had two sessions of class: the morning and afternoon class. Kindergarten was just three hours long. Presently, kindergarten is six hours. Back then, it was the Baby Boomers coming through, so schools were pretty full.
Today, the students start school in August if they turn 5 by September 1st. If they turn 5 sometime between September 2nd and December 2nd, they start what is called transitional kindergarten. This gives them an extra year of preparation before entering kindergarten, and helps to give them more time to mature and prepare for the academic expectations that kindergarten now requires.
I started kindergarten in September 1961. Kindergarten then was mostly play and socialization with some emphasis toward academic readiness. Also, fine motor skills were practiced such as coloring, pasting, painting, cutting, etc. Learning how to read began in first grade. Today, reading instruction begins in kindergarten.
I grew up in San Francisco and received my education entirely through the public school system. Even my college time was spent at SFSU from my freshman year all the way through my post graduate studies. In addition, I have taught in the San Francisco public schools for over 30 years, and my kindergarten teacher was the same kindergarten teacher that taught Governor Jerry Brown. (Her name was Miss Pon.)
I hope this answers your questions regarding the high/low grade split in the San Francisco Unified School District. I believe the practice of grade splitting was stopped in San Francisco sometime around the late 1960s.