In America, the ‘disabled’ (retarded) children used to be (and maybe still are), herded to and from school in a ‘short bus’ which is literally.. a short bus.
—“The idea of ‘riding the short bus’ is used in American English in much the same way that ?????? ??? (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).”—