From Education Week: States Seek Guidance in Face of ‘Opt Out’ Push
Test participation at issue
The No Child Left Behind Act—the current version of the Elementary and Secondary Education Act—requires each school to test at least 95 percent of its students or else the district or state could face sanctions, some as severe as losing Title I money for low-income students. That requirement must be met for all students in a school, as well as for subgroups of students, such as those living in poverty or from racial-minority groups.
States and schools that narrowly miss the 95 percent threshold are allowed to average participation rates over a two- or three-year period to help meet it.