This can be looked at least two ways.
First, you could say that all students in American public schools were the target population. The law called for all students to be at or above grade level in core subjects by 2014.
Second, you could say that the target population was students who do badly in school or are at risk of doing so. The real point of the law was to help them improve. The students who already do well in school did not need the law nearly as much as those who did not. The law made this emphasis clear by mandating that all subgroups of students should make progress each year in a given school. That way, the achievement gap between various groups of students could be narrowed.