To use the standard EL argument: If they did that, the first kid killed 'when lights might have helped even if not legally applicable' will result in a massive lawsuit.
School likely knows the lights arent legally binding all the time, and just has a policy of 'lights on when kids are moving'
ON ANOTHER ASPECT:
If a parking lot is (say) 500 feet by 500 feet....and a bus is somewhere inside said parking lot, how does one define 'passing'? Unlike a 'highway' where there is a physical roadway, do the painted lines within a parking lot now become little roadways?
Or is it just safer to not move anywhere in a parking lot, no matter the size, when a bus is loading/unloading as this would be the safest? Just draw a line perpendicular to the buses axis of travel, extend this to the ends of the lot, and do not cross this line even if hundreds of feet away....