At this point we pivoted back to the original research as it had the most useful statistics we had located.
2017-2018 Transportation Statistics
These are the most current statistics on the website. They state the number of total buses for each state, the owners of the buses, and the number of students transported daily. This would seem the most reliable source available, as school bus would not be on the road transporting students if it was not registered. They also state the total number of school buses in the US (476,150).
Additionally, they break down sales by the US, Canada, and North America for 2010-2019 here.
The numbers were verified by Axios, who put the total number of buses in the US at 480,000 and stated there were still less than 1% electric.
With further research to validate the actual number of buses that run on diesel and other types of fuel, the numbers could be triangulated for the US and the individual states.
Only the project owner can select the next research path.