I poked around and found that Texas was the first state to have a dyslexia law. Here's a link to their handbook:
http://www.region10.org/dyslexia/Documents/DyslexiaHandbook11-10-2010.pdf

In skimming it, I noticed this call out to specific ability discrepencies that imply simply achieving adequate grades is not the measure:
Quote
A. The pattern of weaknesses in a student with dyslexia will reflect one or more difficulties with
low performance for the student’s age and educational level in the following academic skills:
 Reading real words in isolation
 Decoding nonsense words
 Reading fluency (both rate and accuracy)
 Written spelling (an isolated difficulty in spelling would not be sufficient to identify
dyslexia)

Also, I've read references that coping mechanisms that gifted dyslexic children have can start to unravel around 3rd or 4th grade.