Legally speaking, it's hard to say, because it depends on the laws in the particular jurisdiction. There is a wide variation in how these warnings are phrased, and how they relate to local law, for example it may be limited to "when flashing" (which seems to be the pattern in Washington, but that's more a matter of practice than state requirement). Federal Way WA can indicate school speed limits "when flashing" or "when children present", in case
1.) School Children are occupying or walking within the marked crosswalk.
2.) School children are waiting at the curb or on the shoulder of the roadway and are about to cross the roadway by
way of the marked crosswalk.
3.) School children are present or walking along the roadway, either on the adjacent sidewalk or, in the absence of sidewalks, on the shoulder within the posted school speed
It appears that "when present" is a theoretical option in that town, and instead they rely on flashing lights and photo-enforcement. This definition follows from a state administrative rule 468-95-335 that defines "when children present" this way, and the state no longer uses the "children present" standard. In another state / town, the law could be different.