Buy America is a federal procurement requirement. If no federal funds are used, only state laws apply. School buses generally are not federally funded. Federal education funding goes toward actual education, not transportation.

You might recall, many years ago, that San Diego used only local and state funds to build their first modern light rail line. That allowed them to get low-budget Siemens cars instead of American-built (at the time, Boeing), expensive, cars. Dodging federal procurement rules let them save a lot of money. After San Diego and a couple of others ordered Siemens cars, Siemens set up an assembly shop in Sacramento, which has grown considerably since then.