Welcome to the skoolie sickness.
Engine swaps by school districts on gas buses are more likely than a model with a good diesel engine (5.9 and 8.3 Cummins, DT 360 and 466). The gas jobs are just not up to pulling around that kind of weight.
I would Google the two engines that you think it may be. The outside of the block is the same. Only internals are different.. The 366 was used years before the 454. Due to the year of the bus, it's most likely a 454.
This is from wiki
Mark IV engines saw extensive application in Chevrolet and GMC medium duty trucks, as well as in Blue Bird Corporation All American and TC/2000 transit buses (the latter up until 1995, using a purpose-built, carbureted 427). In addition to the 427, a 366-cubic inch (6.0 liter) version was produced for the commercial market. Both the 366 and 427 commercial versions were built with a raised deck, four bolt main bearing cap cylinder to accommodate an extra oil control ring on the pistons. Unfortunately, the raised deck design complicated the use of the block in racing applications, as standard intake manifolds required spacers for proper fit. Distributors with adjustable collars that allowed adjustments to the length of the distributor shaft also had to be used with 366 and 427 truck blocks.