Present means "present in the school." It's not always observable while driving by if you need to obey the reduced limit or not. California does it and I find it absurd.

Many other states setup a flashing yellow light and program the light with the school schedule. Then the limit only applies "when light is flashing." Far more sensible.

This is the one most familiar to me. Usually the signs have flashing orange lights to indicate when they're active, but sometimes not. You generally know roughly when the kids are in school (maybe look at the school?), and follow what other drivers are doing. Things like this are why I think fully autonomous driving basically requires AGI.