The question has arisen whether there are any laws about school bus drivers picking up hitchhikers.
As a matter of fact, yes. It is a violation of Iowa Code section 285.11(2) and (6) to allow a school bus to transport any persons other than pupils and school employees (including volunteers such as field trip chaperones).
Allowing non-authorized persons to hitch a ride subjects a school district to forfeiture of transportation costs AND subjects a superintendent or school board that knowingly permits a school bus to be operated in violation of any school transportation law to conviction of a simple misdemeanor. (Iowa Code section 285.15)