The Collaborative International Dictionary of English v.0.48:

Classmate \Class"mate`\, n.
   One who is in the same class with another, as at school or
   college.
   [1913 Webster]

WordNet (r) 3.0 (2006):

classmate
    n 1: an acquaintance that you go to school with [syn:
         schoolmate, classmate, schoolfellow, class fellow]