Many school districts have policies (age 5 as of X date) set by the state dept of education. See if the state dept of ed in tx has policies on the internet and review them. Maybe talk to the head of special ed (is gifted under special ed in tx?). Research if there has been a precedent in TX for early start and if so, ask for a meeting with state ed people to talk about it. Perhaps the principal really doesn't have the power to make an exception, but find out who does have the power. There have been many challenges recently in other states that have been successful. Good Luck!
P.S. I found our private preschool was about a year advanced of the public schools, so challenging to get into K may not be adequate, depending on your child's skills (can he read a little?). I just had my son skip Kindergarten and go right into 1st, but he did make the date cutoff by 24 days.
