All the major manufacturers made various bus models with some having cargo bay storage built in them from the factory. Cargo bay storage is usually for high school band trips and/or football games when the school is playing at another high school. These buses are more expensive for the school districts to buy. Therefore, these buses are not as common as most school buses. However, with due diligence, they can be found. The best way to determine if a bus has cargo bay doors is to look at the bottom of the bus in pictures or to view the bus personally. If there are large horizontal swing doors below the floor line, then you have a cargo bay bus. Sizes of the bays depend upon how the district ordered the bus.
For example, in Katy, Texas, we have Blue Bird, Thomas, and International buses that are equipped with cargo bays. Yet the older International buses have smaller cargo bays than the Thomas, Blue Bird, or newer Internationals. At the time of the order Katy ISD anticipated a smaller need for cargo bays and ordered those buses based upon that assumption. However, three years later Katy began to experience a large population growth. This compelled Katy ISD to build many new schools. They also ordered a series of new buses at that time with cargo bays twice the size of the previous buses.
So, as I said earlier, the best way to determine if you have a bus with cargo bays is to go visit the bus or to carefully inspect the pictures of the bus before going further.
Hope this helps.
Firearms stand next in importance to the Constitution itself. They are the American people’s liberty teeth and keystone under independence. — George Washington