What about **"an underachiever"**?  

- someone who does not perform as well or work as hard as he or she can.

>Definitions:

>- "one that fails to attain a predicted level of achievement or does not do as well as expected" [MW](http://www.merriam-webster.com/dictionary/underachiever)

>- "a student who does not perform as well as expected or as well as the IQ indicates" [TFD](http://www.thefreedictionary.com/underachiever)