you have described my 5 year old son to a T. Let's just say that between January and the end of April he is now in Therapy for severe anxiety and learnt underachievement and is no longer attending school.
I will do a proper update post as soon as I have the emotional strength to do so.
Once they figure out that they onyl have to do half of what they are truly capable of, they get so angry inside. But some of the higher-level gifted kids don't show it outside of the home from what I have read.
http://www.hoagiesgifted.org/underserved.htm explains it nicely.