Why do kids have to go to school so early in California?
I don't think that you mean early in the day. I teach in San Diego, and most schools start around 8 in the morning. Some schools have a "zero" period that starts at 7 in the morning, but students don't have to take that period. A lot of the time athletes will choose to take a class that period in order to have the last period of the day off. That makes it easier for them to not miss class on days with away games. I taught for a few years in Michigan, and the school that I taught at there had a start time of 7:30 a.m. So California schools are starting later than that one.
If you mean earlier in the year, then you are correct. Most of the schools in my area start around August 15. That's a lot earlier than many other places that start around September 1 and Labor Day.
California schools do this for a variety of reasons. One reason is sports. Playoff dates and schedules are set and basically unmovable. Starting school earlier in the year, guarantees coaches that athletes are on campus. Teams get more consistent practices in if school is in session. An earlier school start helps out athletics.
Another reason to start school earlier in the year is that it guarantees 1st semester is finished before Christmas break. If a semester is 18 weeks long, the school year has to start early enough to finish those 18 weeks before December 20ish.
A third reason is advanced placement courses. AP exam dates are set as well. If a school starts two weeks earlier, then AP teachers get an additional two weeks before the AP exams to prepare those students.