An interesting observation about the high/low system. If the year was evenly divided, one would expect that each class would have approximately the same number of students. But the class that started in September was 2- 3 times larger than the class that started in February.
Two reasons why: children who moved into the city, and students who transferred from the catholic schools were in the September-June tract.
In his book, Outliers, Malcolm Gladwell has a chapter about professional hockey players. A large percentage of these players have birthdays in January to March. Why...young boys in "hockey countries" such as the Canada, enter youth hockey at a very young age. The boys born early in the year have a slight advantage because they are a little older. They show talent very early in their career, and this compounds as they are selected to the better teams and receive better coaching as they progress along.
By dividing the year in two, our SF high/low system might have smoothed out other developmental items.