ASK A COP: What's the Speed Limit in a School Zone When Students Aren't Present?

When the school sign is posted "School Zone - 25 MPH when children are present," does this mean you can go the speed limit e.g. 35 MPH in the school zone if there are no children present on the sidewalks or nearby, or no children in sight even though school is in session.

When I travel on near a school, I always slow to 25 MPH when in I am in the zone but others zip past me at the 35 mph speed limit if there are no children in sight.

Some school areas have a  "school zone 25 MPH when school is in session" which I find a safer sign.

What is the vehicle code for this?

Answer from San Mateo County Sheriff's Deputy Rebecca Rosenblatt: 

California vehicle code 22352(B) states...

Twenty-five miles per hour:

When approaching or passing a school building or the grounds thereof, contiguous to a highway and posted with a standard "SCHOOL" warning sign, while children are going to or leaving the school either during school hours or during the noon recess period. The prima facie limit shall also apply when approaching or passing any school grounds which are not separated from the highway by a fence, gate, or other physical barrier while the grounds are in use by children and the highway is posted with a standard "SCHOOL" warning sign.

For purposes of this subparagraph, standard "SCHOOL" warning signs may be placed at any distance up to 500 feet away from school grounds.

And to keep you slowed, the 25 MPH law also applies to senior centers:

When passing a senior center or other facility primarily used by senior citizens, contiguous to a street other than a state highway and posted with a standard "SENIOR" warning sign. A local authority is not required to erect any sign pursuant to this paragraph until donations from private sources covering those costs are received and the local agency makes a determination that the proposed signing should be implemented. A local authority may, however, utilize any other funds available to it to pay for the erection of those signs.