The Massachusetts state speed limit for a school zone in 20MPH. A signpost like Frono's example above would indicate that the limit is 15MPH during the posted school hours when school is in session. At other times, it'd be 20MPH (in Massachusetts).
Al's original posted picture would be meaningless in Massachusetts, but could be real in another state where the school zone limit is 30MPH, say. Of course, that's an absolutely ridiculous (and funny) list of enforced times. I presume it's a joke, otherwise someone should lose their job.
And, Fro, what was the ticket for?