I saw this thing on an episode on Monster Garage's Monster Nation show. (Maybe we'll see lapeer20 on there sometime) Anyways it's not a school bus but I think this may be the ultimate conversion.
Dobbertin Surface Orbiter
The Dobbertin Surface Orbiter is a hand built amphibious vehicle, constructed from a 1959 Heil milk tanker over a period of four and a half years (14,000 man-hours) by Rick Dobbertin of Syracuse, NY.
The vehicle was designed to circumnavigate the Earth, over land and sea, without any type of support vessel and was well on its way when the lack of funding cut the adventure short.
Even so, the Orbiter has logged in over 33,000 miles on land and over 3,000 miles in the open ocean, with seas up to eighteen feet high.
The Orbiter has been the focus of attention of educators in many countries. It sparks interest in science, math, technology and geography for students of all ages.
The Orbiter was the first amphibious transit of the Panama Canal (February 21-23 1995)
One guys single handed took this milk trailer
and turned it into this: