Driving a school bus in the USA

Since you hold a european driving license, I suggest you acquire an International Driving Permit. And since you’re in Spain, do check the page about “Permiso Internacional” in the Dirección General de Trafico website. It’ll take one visit to your nearest DGT office and 10.30€ (as of 2018).

It’s not needed in all cases, but some states require it:

People who drive in the U.S. must have a valid driver’s license. Some
states require an International Driving Permit (IDP) from foreign
nationals, in addition to a valid license from your own country.
Contact the motor vehicle department of each
state you will drive in
for its requirements.

Furthermore, car/bus rental companies commonly require you to hold an IDP. These might in fact be able to tell you if you need an IDP to rent a bus.

Even though it might not be needed, it doesn’t hurt to do so – it’s just a translation of the fields in your Spanish license card, which can be helpful if you need to show your license to a person who doesn’t speak any Spanish at all. The spanish ministry of external affairs recommends visitors to have one as well:

España no tiene convenio con ningún estado de los EE.UU. sobre validez
del permiso de conducir español en ese país. Por tanto, los turistas o
los españoles en viaje de negocios en EE.UU. deberán obtener en
España, y previamente a su llegada a EE.UU., el permiso internacional
de conducción, que tiene validez de un año y es reconocido por EE.UU.
En caso contrario, podrían ser sancionados por carecer de permiso de
conducir reconocido por las autoridades de tráfico locales.

As other answers point out, laws vary by state. It’s up to each state to recognize a class D (bus) european license + IDP as a document enabling you to drive buses – or not. I suggest you find the DMV office for the specific state(s) you’re visiting and ask there.

Federal law mandates that a commercial driver’s license or CDL, issued by a state (or territory or the District of Columbia) is required to operate

Class A: Any combination of vehicles which has a gross combination weight rating or gross combination weight of 11,794 kilograms or more (26,001 pounds or more) whichever is greater, inclusive of a towed unit(s) with a gross vehicle weight rating or gross vehicle weight of more than 4,536 kilograms (10,000 pounds) whichever is greater.

Class B: Any single vehicle which has a gross vehicle weight rating or gross vehicle weight of 11,794 or more kilograms (26,001 pounds or more), or any such vehicle towing a vehicle with a gross vehicle weight rating or gross vehicle weight that does not exceed 4,536 kilograms (10,000 pounds).

Class C: Any single vehicle, or combination of vehicles, that does not meet the definition of Class A or Class B, but is either designed to transport 16 or more passengers, including the driver, or is transporting material that has been designated as hazardous under 49 U.S.C. 5103 and is required to be placarded under subpart F of 49 CFR Part 172 or is transporting any quantity of a material listed as a select agent or toxin in 42 CFR Part 73.

The specific classification of license you need is dependent on the state, but you are required to have a CDL with sufficient endorsements if the school bus is designed to seat 16 or more passengers, even if there are a fewer than 16 of you.

If your group is indeed that large, you can rent large vans or mini-buses configured to seat up to 15 (e.g. Ford Transit, Mercedes-Benz Sprinter). Just search for "15-passenger buses" or shuttles, designed specifically to fall under the limit. If you have your heart set on the iconic yellow school bus, a quick web search did turn up a few companies that have configured school buses to seat 14 or 15, though these seem to be aimed at in-town excursions (e.g. youth groups, bachelor parties), and I cannot say whether any would be willing to rent you one for a cross-country trip.

I doubt very highly that you will be able to find anyone who will rent or lease a school bus to you unless the driver has a driver’s license issued under federal commercial driver’s license (CDL) regulations with an S endorsement.

According to the Federal Motor Carrier Safety Administration, there is reciprocity with Canada and Mexico, but if you have a foreign license that allows you to drive buses from another jurisdiction, you might be able to get a “temporary waiver” of the federal CDL requirement. These are said to be “rare,” however.

Each state in the US has its own regulations regarding licensing for drivers.

In most states (if not all), you will need a special category of license to operate a bus with passengers. Here are the classes of driver’s licenses in Maryland, for example. You would need CDL Endorsement “S” in Maryland (not on Rt. 66, obviously, but it’s an example).

If you are going to be driving in multiple states, you may need licensing and registration with the federal government as well (e.g., the US Department of Transportation), but I’m not sure about that.

Renting a bus with the intention of driving passengers, will have its own set of complications, since the liability insurance will be quite different than just driving yourself and a few other passengers. Be aware that the rules of the road are different (school buses must stop at all railroad crossings even if the lights are not flashing, for example).

Finally, unless you are actually driving a bus for a school (and are an employee of a school district or a contractor for one), you will probably not be permitted to drive a vehicle with “SCHOOL BUS” markings.