Now, do you want an answer from someone who knows what he's talking about? I've answered this one countless times.
1. Small school buses having a Gross Vehicle Weight Rating of 10,000 pounds or less(Head Start-style) are required to have lap belts, and have done so since
April 1, 1977; they can have seats spaced as far apart as the vehicle size will allow.
The reason for the belt law in these vehicles was enacted because, in 1977, these
vehicles were basically cargo vans with school bus-style windows installed.
2. Now to large buses. They are built to an entirely different set of
specifications, including the seat spacing. The seat spacing is
controlled in order to provide a "cocoon" for the passenger, referred
to as compartmentalization. They are also required to have more
emergency escapes. Lap belts are currently required on ALL school
buses in New York, and new buses in New Jersey, and Florida;
California and Louisiana currently require lap and shoulder restraints on all seats
on new school buses purchased after July 1, 2004. Currently, the federal government is looking at increasing the seat back height from 24" to 28" to provide a
higher "cocoon". They are also looking at increased fire retardance inside
the bus.
Motor coaches are not (yet) required to have belts, but Greyhound is starting to order all of their new buses with lap and shoulder restraints.
WHEW! I'm done. I'll step down from my soapbox now.