I had the somewhat same experience as poster 10:00. My DD missed the deadline for K by 7 days. She was a tall, mature girl. So she went into K already knowing the whole year's cirriculum. She already know how to read, write, alphabet and how to sit in a chair and interact with others. The teacher told her that she needed to relearn all this. So she did. She repeated the work in so many grades. Redid the first in third. Redid the 6th in 7th. Bored out of her mind. Finally I took her out of public and put her in private where she could be more challenged. Looking back (now in college), she would have been much better off to start K a little younger. Being ahead in the subjects did not help her. Repeating everything did not help her. She thought the childs version of WTF. (Yes, she did test gifted)
As 10:00 said, it made the teacher mad, or the teacher simply used her as an assistant. Then the teacher would get mad that she was not an adult assistant (at 8). We could not afford private, but she desperately needed to learn at her own pace, and get some recognition for that, so we made the sacrifice and sent her to private. In retrospect, she would have been better off to start early, not late.