Consider a developmental optometrist for children
My daughter had vision problems that were corrected with vision therapy working with a developmental optometrist. They deal with problems involving how the eyes work together. These are not problems with the eye itself, but with control of the eyes. They can be associated with dysautonomia, so it's not uncommon in PWCs.
One problem is
convergence, which is the eyes moving together and apart to see objects at different distances. Her eyes would move too far to the center, then sort of wobble. This made words blur on the page, and she didn't space properly when writing.
Another problem is
tracking, which is the eyes moving together while scanning from side to side as you do when you read. One of her eyes would lag behind, then jump to catch up.
All of these kinds of "skills" are learned by children, and because they don't know any other way of seeing they won't complain. In younger children, this could explain a lot of problems in school.
My daughter had headaches, and I had heard a discussion of these kinds of problems so I had it checked out. The pediatrician's quick eye exam, and even an ordinary exam by an optometrist or opthamologist, may not find these problems. You may need to see a developmental optometrist.
Here is a list of symptoms of developmental vision problems:
http://covd.org/Home/Parents/SymptomsChecklist/tabid/279/Default.aspx
Although they talk about this causing learning problems, my daughter was a good student and a voracious reader. It was just a lot harder for her than it should have been. Homework took forever, and was very tiring.