The NY State law is that even with a median, if there is a flashing red light school bus stopped ALL traffic must stop. My friend learned this the hard way when she was ticketed for passing a school bus with red flashers, the equivalent of two small blocks across from her (traffic in either direction was divided by a median and each side of the median was the size of a three lane highway). She tried to fight the ticket only to learn that she was in the wrong.
As to the ambulance – if the persons not allowing it to pass were to discover that their loved one died in the ambulance because it could not get to the hospital in time, they would feel very sorry. We have to think of every ambulance in that way, and maybe we won’t be so thoughtless AND BREAK THE LAW.