"Never" is a big word in education, especially as it relates to a probable or identified GT child. I think this thread shows numerous examples of parents of GT children who are glad they did and glad they didn't red-shirt their child.
It IS a very difficult decision, so little hard evidence with that particular child to base a the decision on, I understand how any parent goes back and forth on this. No doubt we've all made decisions about our GT child we'd like do overs on!
What I have learned is this, 95% of the time when I follow my parent's intuition, I'm glad I did. Nobody knows your child better than you.
