The question What do, "X speed limit while children are present", street signs mean, specifically? has an upvoted answer indicating that the "When children are present" speed limit varies by jurisdiction.
This question, in response, is specifically for one jurisdiction: California. What is the meaning of "when children are present" on speed limit signs (non-flashing)? Does it refer to only tehe periods where children are visibly present? Or school hours? Or when there is an after-school football game? How is one to ascertain the presence or absence of children from the school under Californian law?