overachiever

from WordNet (r) 3.0 (2006)

overachiever
    n 1: a student who attains higher standards than the IQ
         indicated

from The Collaborative International Dictionary of English v.0.48

overachiever \overachiever\ n.
   1. A student who attains higher standards than expected based
      on the IQ score.
      [WordNet 1.5]

   2. A person whose achievements exceed what was expected based
      on his/her background, education, or mental ability.
      [PJC]