The intense efforts of No Child Left Behind to bring the performance of the nation’s worst students from unacceptable to adequate may be the same force stopping the movement of its brightest students from excellent to extraordinary, according to a report released Wednesday by a Washington-based educational research group.

Since 2000, the start of the No Child Left Behind era, researchers from the Thomas B. Fordham Institute found test scores for students in the lowest 10 percent soared, while scores for their peers in the top 10 percent “languished.”