the prison busses here in columbus use hard plastic seats, there are coach style seats out there that were made for school busses. but are rare to find.. theres an IC bus in my storage unit area that has the side by side coach style seats.. though they dont look near as nice as say trip-busses like MCI's or Vanhools, etc..
MOST typicasl school bus seats are 2 legged, one side mounts to the chair rail.. and on both my busses that rail is at the same height and the screws are equal distamce.. one is a bluebird, one is a carpenter..
the other 2 legs just go through the floor and in fact are spaced such that the miss the ribs and frame.. there is a decent size washer on the bottom of each seat bolt under the bus.. school bus seats came in all kinds of widths and school systems seated them for different pitches.. standards were followed for many but sometimes a band bus might be ordered with more distamce between seats knowing the passengers would be in thick uniforms.. wheelchair busses often had narrower seats and wider aisles ..
from what I can tell.. the body was built, floors installed, then seats fitted.. then body placed on chassis.. the real issues re-seating a bus come about from trying to get to spots under the floor.. blocked by chassis items like fuel tank, air tanks, air-conditioners, etc.. but drilling the frame rails isnt necessary to install your new seat.