WordNet (r) 3.0 (2006):

drop out
    v 1: give up in the face of defeat of lacking hope; admit
         defeat; "In the second round, the challenger gave up" [syn:
         drop out, give up, fall by the wayside, drop by the
         wayside, throw in, throw in the towel, quit, chuck
         up the sponge] [ant: enter, participate]
    2: withdraw from established society, especially because of
       disillusion with conventional values; "She hasn't heard from
       her brother in years--he dropped out after moving to
       California"
    3: leave school or an educational program prematurely; "Many
       students drop out because they are not prepared for our
       challenging program"