GP is saying that reading the sign is insufficient to determine whether it is a school day. You have to either guess based on the presence of students or busses, the lights being on, etc., or you have to source the school calendar somehow.
I don't know how it is there, but here those signs near schools light up and blink during school hours (really can't miss it). And for signs that do not, I think school days are pretty fixed, shouldn't be difficult to program... and a default of just slowing down would be just fine too.
