The Collaborative International Dictionary of English v.0.48:

Correspondence school \Cor`res*pond"ence school\
   A school that teaches by correspondence, the instruction
   being based on printed instruction sheets and the recitation
   papers written by the student in answer to the questions or
   requirements of these sheets. In the broadest sense of the
   term correspondence school may be used to include any
   educational institution or department for instruction by
   correspondence, as in a university or other educational
   bodies, but the term is commonly applied to various
   educational institutions organized on a commercial basis,
   some of which offer a large variety of courses in general and
   technical subjects, conducted by specialists.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

correspondence school
    n 1: a school that teaches nonresident students by mail