I don't think IQ is stable until a child is about 8 or 9. So many kids who appear very advanced at age 3 will not still be advanced at age 8. And some kids who appear average at age 3 do indeed end up "gifted". My son was one of these--he had developmental delays and his IQ shot up about 30 points in the last 2.5 years.
I do believe that those kids who start out at the top of the class in kindergarten or first grade tend to stay there, esp. if the teacher gives leveled materials and differentiates instruction. If a kid starts out in the "low group" and everyone else in the class is doing more advanced work, it's going to be pretty hard for him to catch up. The same is true of those in the "high group". If they are given 20 spelling words and others in the class are given 10, then they will likely stay highest in spelling. Even if they are really not that smart.
My DD was grade accelerated in the middle of kindergarten and quickly caught up and surpassed the kids in the next higher grade, so of course there are exceptions to this. She's never had a real IQ test, I don't know if she's officially "gifted" but I would guess the fact that she was able to do that is pretty good evidence. I also think that those kids who are given work that is above their grade level are going to be those that continue to score best on achievement tests. If I stop trying to give DD enrichment stuff, she will likely slip a bit. Not go down to average, but other kids will surpass her if they do extra work at home or are given special opportunities in the classroom or other schools. Our school/district is terrible about allowing kids to work ahead in math so I'm surprised that anyone in the district is scoring in the upper percentiles for math on nationally normed tests. The ones that are are probably being given instruction in advanced math at home. Those that aren't, could appear to be plateuing.
