I have loaded the SchoolZonesQLD.gpx from this site with the extra alert set at 300m but on testing at acouple of local schools, the alert warning comes far too late. Well into the 40k spped zone.
I can only assume that one SAT point is nominated for a school zone and likely be in the middle of the zone, or actually where the school is.
I will test with another file selected at say 700m for the alert.