Stop Wasting Money Teaching Millions of Students Content They Already Know
Thomas B. Fordham Institute, Advancing Educational Excellence
September 19, 2016
by: Plucker, Makel, Rambo-Hernandez, Matthews, and Peters

Originally Posted by article
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consistent evidence that very large percentages of students perform above grade level.
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estimate that 20–40 percent of elementary and middle school students perform at least one grade level above their current grade in reading, with 11–30 percent scoring at least one grade level above in math.
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large percentages of students performing well above grade level—more than one grade level ahead.
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8–10 percent of Grade 4 students perform at the Grade 8 level in reading/English/language arts
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2–5 percent scoring at similar levels in math.
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one out of every ten fifth-graders is performing at the high school level in reading
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nearly one child in forty at this age is performing at the high school level in mathematics.
Using midpoints of percentage ranges given, it seems that:
20% of kids are at least one grade level ahead in math,
2%+ are 4 years advanced.
30% of kids are at least one grade ahead in reading,
10% are 4 years advanced.

This feeds the obvious problem of what a child doesn't learn when underchallenged (as discussed in a recent thread).

IMO, these percentages also help illustrate that gifted programs which teach one year ahead often do not meet the needs of gifted students. A one-year grade acceleration (grade skip) may also not meet the needs of some gifted kiddos.

Related thread: Johns Hopkins: How can so many kids be invisible?