kikib, I'd suggest three things:
1) Does her output improve with scribing or typing? If it does, that might indicate there's either a visual or fine motor integration issue going on.
2) Do supports like sitting with her and scaffolding written assignments help, or graphic organizers, or repeating assignments help?
3) If #1 doesn't happen and #2 doesn't seem to help (output is still difficult, disorganized, etc), I'd consider a speech language eval. Kids with written expression challenges often benefit from working with an SLP on how to generate ideas, how to organize them etc. Another test that might help in addition to an SLP eval is the TOWL (Test of Written Language).
I'd also keep an open mind re considering that this might not be an ADHD-specific issue, but rather a challenge that is masked by the ADHD when she's not on meds - sometimes with 2e kids, multiple challenges exist and when they are young in particular, it's necessary to accommodated successfully for one first before realizing that another exists underneath. Hope that makes sense!
There might also be some clues in previous achievement testing if you had any to go along with her ability testing (WJ-III, WIAT, etc). If she had achievement testing and you have a report with subtest scores, look to see if there were high vs low scores or if the subtest scores were all relatively even.
Best wishes,
polarbear
ps - The reason I included "repeat" in #2 above is that in my children's elementary years the LA curriculum would bounce around from one type of assignment to another without ever truly repeating a skill. If her class is already set up with a logical writing curriculum where skills are reinforced, then you can ignore the "repeat" suggestion
