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.
Interesting that you brought up the learnt underachievement. That is something the tester we are going to use this summer suggested could be the cause of her faltering trajectory around the time she entered pre-k at age 3.
Now whether or not that is the cause of her temper tantrums and behavior I'm not sure. She is definitely an intense child at times. She hurt her sister again yesterday when they were arguing. I am at a loss as to how to punish her for this b/c sending her to her room and apologizing for her actions doesn't seem to be doing anything. And she isn't using the code word we came up with for when she is angry or frustrated.
And her night terrors and sleepwalking are picking up speed and happening every night again. It's not as bad as last fall when it was 2+ times a night, but still exhausting for me. I think it has to do with the bugs she has been so interested in learning about lately as she will sometimes talk about bugs during her terrors/sleepwalking. She never remembers but she is anxious about going to sleep and says she feels bugs crawling on her or that she's worried there are bugs in her room. Yet she won't stop reading the bug books, hunting for bugs, or playing with her pet click beetle.
ALL our issues accelerated the longer he was at school - weekends were pleasant until Sunday afternoons. Then it would start again. School breaks would be wonderful until he found a calendar and saw less than 5 sleeps to school. It affected everything: appetite, sleeping, bedwetting, affection levels. At the very end even his physical abilities sometimes regressed when the anxiety was exceptionally high - he would walk like our toddler with his hands up by his shoulders, his speech would degenerate, he would whine and moan instead of talking.
And yes, it affects EVERYONE in the house.
They cannot apologize for it - it's not malicious - it's their way of crying out for someone to do something to change the trap they feel they are in. Problem with starting "school" from age 3, they cannot even explain why the academic fit is all wrong for them, they can't verbalise the boredom or anger - simply because it has most likely been the only clear association they have with nearly all of their schooling experience.
On the underachievement thing: It's very real!