I have an almost 7-year-old son who has an IEP and I was the one who ended figuring out his problem. The schools are overwhelmed, and they are not versed in the unusual learning disabilities that few have (my son has auditory processing disorder which is as unusual as a visual processing disorder).
I would strongly recommend you take your daughter to an optometrist or opthalmologist who specializes in visual issues in children. We took our son to Anne Simon in Torrance (http://annesimonmd.com/) to rule out any visual problems. Also, The Learning Gym (http://www.learninggymusa.com) in Manhanttan Beach does an excellent screening of learning issues, including visual issues, and they could tell you if you need to investigate further.
I have to tell you, I suspected auditory processing disorder in my son for nearly a year before we got the final diagnosis. Before I suspected it he had been in special ed for 3 years and NOBODY mentioned that it might be a possibility. These are teachers and aides who specialize in special ed, and they didn't even pick up on it. When our son was mainstreamed, the school's IEP team was adamant that I was wrong. They flat out told me they had tested him and he showed no signs of APD. But my intuition kept telling me I was right, they were wrong. In the end, my intuition did prove accurate when an audiologist confirmed he not only had APD, he had it pretty severely. And on the a side note -- my son passed every school and pediatrician hearing test since birth, but when we took him to an audiologist, he was found to have a mild hearing loss. The schools and pediatricians aren't specialists.
The moral of the story is... don't give up. No matter what the school tells you, even if it's the school psychologist or a special ed teacher, if your intuition says otherwise, follow your intuition. I have spoken to so many moms who have walked the same path, and if they had not followed their intuition, their kid wouldn't have gotten help.
One final note, get yourself a copy of The Mislabeled Child by Drs. Eide. This book goes through every single learning disability, including visual processing disorders, and it also will tell you the ones that look the same, so you'll know what to test for. These two neuroscientists specialize in dyslexia. This book helped me figure out what was going on with my son.
Sorry I don't have any advice about holding a child back. Haven't gotten to that age yet. :-)
Best of luck,
B.