The district in which I live is the worst of both worlds, it both limits the number of students admitted to the 1 GT class to 15 students and highly considers achievement testing and teacher recommendations, though ability testing is also a component. Redshirting even a slightly above average student can give that child a huge advantage in GT entrance.
FWIW, none of this affects my child since he attends a private school (where redshirting is just as bad, but the whole curriculum is advanced anyway). I just don't think it's fair to the kids in our public school to have parents unfairly gaming the system. It's also too obvious who the redshirted kids are because they have to play league sports (also huge in my area) with their agemates, so other parents clearly see what's going on. Absent extraordinary circumstances, it just doesn't pass the smell test for me. Just my personal opinion, and obviously there are many different points of view on this subject.
