There's a huge difference between the line-of-sight available to a driver and a cyclist as well as their vehicle's respective handling. A driver trying to slide past a school bus is making a serious gamble, as their bumper is jutting out yards ahead of their actual line-of sight; if a child is about to run out from behind the bus, the driver will be closer to that child by the time he sees it than the cyclist will (as his head will be at the same location as the cyclist or worse, but the front bumper of his car is yards ahead). On a bicycle, I can easily stay well away from the bus and have a huge margin of error between me and the bus as well as an excellent line-of-sight. A driver is limited by the bulk of his vehicle and can't get as far right as a bicycle, so both his reaction time (the child has less distance to run from behind the bus) and his line-of-sight (being further right provides a steeper angle) is vastly inferior. Not to mention that drivers tend to have a under-appreciation for the speed of their vehicle and its associated force.

In this respect a cyclist and driver are an orange and apples comparison, and I see nothing wrong with a cyclist passing a school bus, given that the cyclist does so in a cautious, vigilant, and controlled manner. Fixie kid whipping past the kindergarten bus with a locked rear wheel because it's cooler that way probably doesn't fit the above considerations.