DD is 9 and this has become a major issue since she is falling farther behind the grade level standards requiring stories, journal entries, reports, etc. We asked for a school eval and they denied any services because her scores on standardized achievement tests (which don't measure extended writing ability) are fine. She is able to use the giftedness to compensate on these tests. With DD, I think the problem is that she has poor executive functioning and she has no idea how to organize her thoughts.
If you have an ipad, you can try the app Inspiration Mind Maps which utilizes graphic organizers. There are other similar apps/programs.
Also, the ipad has built in dictation/speech-to-text. My concern with this is that if she uses it too much she will not learn to actually "write", but if the point of the assignment isn't mechanics, spelling, etc. it can be helpful. It allows them to edit what they have said and what appears on the screen.
