I don’t know about this situation, but around here school bus drivers seem to take great pleasure in keeping their stop signs out for extraordinary lengths of time until every kid in the bus has sat back down.
The most annoying thing busses do however is when they stop in front of seldom used railroad spurs going into local businesses and just sit there for fifteen to thirty seconds. These are not true railroad crossings just tracks going diagonally across a busy highway into the yards of a local businesses for loading and unloading purposes. I went to many motor vehicle accidents where cars ran into the back of busses which were sitting in the middle of a lane of the highway for these spurs. This is actually a divided highway that resembles an interstate. At this point some of our transit busses are now doing the same thing.
You complain of school bus drivers not turning off their signs until all kids are seated. That is a policy of all school bus companies that I know of in most jurisdictions. and concerns the SAFETY of the passengers. It is also likely the LAW in most jurisdictions! Why? Because a child can easily fall if a bus takes off before he/she is seated. In addition, the interior lights on the bus turn off when the door is closed, so the doors are left open until the child is seated, so he/she can SEE to be seated. As sunrise was 8:13AM yesterday in Calgary and ALL my kids are picked up BEFORE 8:10AM, the earliest at 7:15AM, I do not shut the door until they are all seated and my four way flashers and strobe remain on until then, also.
AFA your comment about stopping at ALL RR crossings, that is THE LAW! (Unless the municipality passes a bylaw, like Calgary.) If car drivers are too stupid to wtch out for buses, that's THEIR problem! It's not like the buses are hidden! They are big, and yellow, and have yellow reflective tape on them. They also have red flashing lights and large signage that says something similar to "STOP WHEN RED SIGNALS FLASHING". Many also have strobe lights!
If a driver is too stupid not to read or comprehend such signage, well, maybe they shouldn't be driving. Even my 20 passenger special needs buss weighs over 4,300kg (9,500lb), so if you run into my rear end when I'm stopped, likely the most damage to me or 'my kids', might be tiny lacerations from flying safety glass. AFA the bus? A little paint damage whereas your car most likely will be sold for scrap.
In case you are upset about the speed of school buses, in Alberta, like many jurisdictions, the speed limit for a bus bearing a 'SCHOOL BUS' sign is lower than other vehicles. (For example, the speed limit on expressways in Alberta is 100-110km/hr. For a 'SCHOOL BUS' it is 90km/hr.)
When it comes to drivers running into school buses, usually, 
I believe that is a law that they must adhere to. Most states have it. If not, there are bus company regulations that require them to do so.