To figure out if this sign has any force of law we need to know where it is posted.
The term "school zone" is only defined in Ohio law under ORC 4511.21. And the definition only applies to that particular section.
"School Zone" is defined in federal law.
18 USC 921 defines
(25) The term “school zone” means—
(A) in, or on the grounds of, a public, parochial or private school; or
(B) within a distance of 1,000 feet from the grounds of a public, parochial or private school.