A before-school exercise program started by three Natick mothers is the Northeast regional winner of the Active Schools Acceleration Project, a national competition that seeks the most creative and effective programs that promote physical activity in schoolchildren. Kathleen Tullie, Jennifer Lawrence, and Cheri Levitz founded BOKS, formerly known as FitKidz, in 2009. Since its launch in Natick schools, the program has won the backing of the Reebok Foundation and expanded to 80 schools across the country, including elementary schools in Boston, Washington, D.C., and New York City. BOKS representatives were in Washington on Wednesday to accept the award, which was given to nine programs from around the country.