My husband could be in Mensa if he wanted. He didn't get interested in reading until after 2nd grade, couldn't read actually. His grandmother say him in her lap and read the Book of Mormon to him over the summer.
He just had no interest in it until his brain decided to put it all together and like it.
If she's that far behind she should really be in summer school to do a daily reading program.
Also, how are they testing that she's behind? Is it because she doesn't like to sit and read? Do they make her read during a test to see what she comprehends or how many words she covers? What test says she's behind? How are they defining this category?
If she just hasn't read as many books as her peers then she may be reading fine, she may just not like what she's reading. If you've tried everything then she just isn't interested yet.
If she can read the book for science and math and the other topics she likes then she "can" read and comprehend quite well when she is interested.
Perhaps finding some books on the subjects she likes would get her reading list more populated.
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If she's not reading those text books well either then I'd take her to an eye doc who does testing for eye/brain disabilities. My grandson had bad vision. No one took him to the eye doc before he was in foster care. His vision was so bad in his right eye that his brain was no longer seeing any images it tried to send to it.
The doc did an evaluation and determined he needed eye therapy. He gave him some glasses that looked much like those that Nicholas Cage wore in National Treasure to look at the back of the document.They had colored lenses, they have sections that flipped up and down, they did a lot of different things.
He was supposed to do his eye exercises every day a prescribed number of times, the doc put glasses on him at this time.
When he went back to the doc his vision was better and his brain was accepting signals from his eye. After continuing the eye exercises for another few months he was able to see normally and not even have to wear glasses anymore. The exercises retrained his brain.
So taking her for an eye exam for disabilities like this is a good idea if she is having trouble reading all text. It could be any number of things like Dyslexia too.