JMO, but I think the US education is behind many other developed countries. The curriculum especially at the elementary to middle school levels, are too shallow and too slow for many kids. The reason why there are a lot of kids performing above grade level is because the material is too easy. Kids that are struggling to keep up should be placed in a different track or slower paced education. The rest of the kids should not have to suffer through tedious repetitions or explaining their work when they already clearly know the answers.