While we're here talking about physics: another big reason why buses are safe are because of their high mass. I was in one accident while on a school bus - another car hit the bus head-on while the bus was stopped. The kinetic energy of the 1 ton car travelling at say 30 mph - 1/2 * m * v^2 - was absorbed by the bus. I won't step through the math, but because of the high mass of the bus, the resulting velocity of the bus+car was quite low (it also helped that the change in velocity was mitigated by brakes/tires/friction, as the driver was ready for the impact).

In any case, as a passenger on the bus, the result of the impact was quite minimal - there were no injuries on the bus, and the bus actually drove off afterwards (with no passengers and a bit of sheet metal scraping the pavement).

Granted, it's a different story if the bus hits a tree or is hit by a tractor trailer doing 70 mph.

Another reason why buses are safe? The bus is operated by a trained professional, most of which have years of experience driving the bus for hours every day.