The U.S. Environmental Protection Agency (EPA) offers grants up to $25,000 per vehicle to help pay for a new school bus and up to $6,000 per vehicle to pay for vehicle retrofits. Regional, state, or tribal agencies including school districts and municipalities or private entities that operate school buses under a contract with these organizations are eligible for these grants.
Grant details are available at this website: https://www.epa.gov/cleandiesel/clean-diesel-rebates .