A study of the impact of participation in the 21st Century Community Learning Centers after school program on students' supervision, after-school activities, homework and tutoring activities, academic outcomes, behavioral outcomes, safety, developmental outcomes, and parent involvement, with subgroup analyses by grade level, reading level at baseline, behavior problems at baseline, student race and ethnicity, student gender, and household type, based data from 2,308 elementary students in 12 school districts and 26 afterschool centers