Fish in a Tree by Lynda Mullaly Hunt

Ally has been smart enough to fool a lot of smart people. Every time she lands in a new school, she is able to hide her inability to read by creating clever yet disruptive distractions. She is afraid to ask for help; after all, how can you cure dumb? However, her newest teacher, Mr. Daniels, sees the bright, creative kid underneath the troublemaker. With his help, Ally learns not to be so hard on herself and that dyslexia is nothing to be ashamed of. As her confidence grows, Ally feels free to be herself and the world starts opening up with possibilities. She discovers that there's a lot more to her--and to everyone--than a label and that great minds don't always think alike.