Sometimes I see speed limit signs which revert to a higher speed. But you are right: it's not always clear. It's not like a construction zone which indicates an end.

These signs exist already. You need to ask your local or state reps to look into this. Kentucky has these already.

The one thing the world doesn't need is signing that is unnecessary. Too many signs tends to make all signs less noticable.

By not placing signs that designate the end of a speed zone or school zone all drivers are somewhat more aware of their surroundings. So what if they drive a bit more cautiously after going through a signed school zone.

The signing is designed to make you slow down in an area where children are present. As children tend to be present nearly everywhere. . .is it a bad thing that drivers exiting a school zone drive a bit more cautious until they are certain they are out of the school zone?

That, and, most people are able to discern the end of a school zone by observing the sign that faces the opposite direction.

Those 24" X 48" signs are kind of hard to miss.

Update:

The traffic engineer where I live is considering having signs installed that designate the end of the school zone. Not because people don't know where they end. . .but because the police department is uncomfortable ticketing people for speeding in the school zones because the zones end limits are not 100% clearly defined.

Seems like a big waste of resources. All the police need to do is move the radar to the beginning of the school zone. After all, the speed limit begins where the sign is, not when you can see it.