Okay, so if we're setting aside the hypothesis that ADHD is the culprit...
I had a somewhat similar experience with a gifted foster child who was falling behind and in danger of being retained in the third grade. She refused to do her reading assignments due to learned helplessness (she was told she was dyslexic) and because she'd been able to manipulate her way out of doing them before, through various strategies of being charming, crying, etc.
The second part was easy enough to resolve - we simply let her sit at the dining room table and not participate in any of the rest of the family's activities until her work was done, consistently and non-negotiably. But left on her own, she'd simply fidget for hours, try various ways to get attention, and just plain rage. Because that was never going to resolve the very real tears and howls of frustration that were coming from her learned helplessness, which was the reason she avoided doing the assignments in the first place.
To make a long story short, I sat down with her to do the reading assignments together, quickly discovered that dyslexia had nothing to do with it - she was attempting to read via whole-word recognition, and all she required was a crash course in phonics. So I read along with her, wrote out mistaken words on a white board, drew lines through the syllable breaks, and guided her through vowel sounds. As would be expected with a gifted child, she made rapid progress. We celebrated her triumphs together, and that sense of learned helplessness began evaporating under rays of confidence.
So the formula for success in this case were:
1) Consistent enforcement.
2) Investigation of source of perceived inability.
3) Scaffolding and building of skills and confidence.
