What gave you the impression that the principal disliked the handicapped? From what I can recall, the mother wanted a decent education for Forrest in public schools, but the IQ tests for Forrest placed him as a special needs person, thus declining him admittance and forcing Forrest to go to a special needs school. The mother took it upon herself to sleep with the principal in order to get Forrest in and it worked. Does this happen? I wouldn't doubt it.
In any real world scenario, what Forrest's mother did would be negligent parenting because Forrest obviously can't learn at the pace of the rest of the children, thus making him a social outcast as well as a failure in his subjects. I don't recall if they actually touched on the grades he received, but no doubt he would be failing. They also don't touch on whether the mother has to sleep with the Junior High principal or the High School principal to keep him in the public schools, which is kinda funny.
The short of it is that I don't think principal had any bias towards the handicapped, he was only trying to enforce the standards set by the state. I don't think Americans have ever had any problems with the disabled, but if you're a different color or gender, watch out.