How's the weather in late late September

This one group lists temperature in a rather generic sense across the year.

http://ellasvacationrentals.com/california-weather/

https://www.cityrating.com/weather-history/california/

So, it can be mid to high 70s if typical weather prevails. It generally has been hot in California around the first month or so of school. But you are toward the end of the month, which should be a bit cooler.