When children are present

Another piece of vagueness that I drive past every day is a sign restricting the speed limit "When children are present." Well, it's a school. Children are present from 8-4 or something like that. What if there are evening programs? Is there summer school? How can I know these things.

After a web search, I found that I wasn't the only one with this kind of question. Here's what the California driver's handbook says:

Within 500 feet of a school while children are outside or crossing the street, the speed limit is 25 mph, unless otherwise posted. Also, if the school ground has no fence and children are outside, never drive faster than 25 mph. Always drive more carefully near schools, playgrounds, parks, and residential areas because children may suddenly dart into the street.

I think that means unless students are going to or from the school, I can go the regular speed limit. Most cars don't slow down and I expect to be creamed if I do.

p.s. I wanted to use the word "vaguery" but neither miriam webster or the OED would approve.