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]

Moby Thesaurus II by Grady Ward, 1.0:

84 Moby Thesaurus words for "classmate":
   ace, amigo, associate, bedfellow, bedmate, birthmate, bosom buddy,
   buddy, bunkie, bunkmate, butty, camarade, chamberfellow, chum,
   clubmate, colleague, comate, companion, company, compeer, comrade,
   confrere, consociate, consort, copartner, copemate, copesmate,
   couchmate, cradlemate, crony, cupmate, fellow, fellow student,
   general partner, girl friend, gossip, grade schooler,
   high schooler, infant, intermediate schooler, jailmate,
   kindergartner, mate, messmate, nursery school child, old crony,
   pal, pard, pardner, partner, pewmate, playfellow, playmate,
   prep schooler, preschool child, preschooler, primary schooler,
   roommate, school child, school kid, school lad, schoolboy,
   schoolfellow, schoolgirl, schoolmate, secondary schooler,
   secret partner, shelfmate, shipmate, shopmate, side partner,
   sidekick, sidekicker, silent partner, sleeping partner,
   special partner, tablemate, teammate, tentmate, watchmate, waymate,
   workfellow, yokefellow, yokemate