The idea of 'riding the short bus' is used in American English in much the same way that 'æåëòûé äîì' (yellow house) is used in Russian as a reference to mentally retarded children. United States public school systems have an extensive bus program to transport children to and from school. Regular buses hold large amounts of children. Shorter buses are typically used for "Special Education" (i.e., the mentally retarded) public school students who are typically educated in different facilities.
Oh don't mind Jimmy here being late - Marina and I took a cab, but Jimmy's a little slow so he had to wait for the short bus.