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"

Moby Thesaurus II by Grady Ward, 1.0:

52 Moby Thesaurus words for "drop out":
   abandon, agree to differ, agree to disagree, back out,
   be at variance, be in dissent, beg off, beg to differ,
   break bounds, break step, cry off, depart from, differ, disagree,
   disagree with, discard, discord with, dissent, dissent from,
   divide on, evacuate, forsake, go back on, jettison, jilt, leave,
   leave behind, leave flat, make waves, maroon, nonconform,
   not agree, not comply, not conform, oppose, opt out, protest,
   pull out, quit, quit cold, renege, rock the boat, say goodbye to,
   secede, stand down, take exception, take issue, take leave of,
   throw over, vacate, withdraw, withhold assent