For the first time, all kindergarten students in Prince William County, Va., will have full-day sessions. Fairfax County, Va., is adding full-day kindergarten in 21 schools, a shift that makes the program available in about 70 percent of county elementary schools. Students in Washington, D.C., Arlington County and Alexandria have full-day kindergarten. Maryland made it mandatory this fall. Educators say half-day classes are becoming a thing of the past because young students need more than a few hours of class each day to master the building blocks of literacy and math before first grade.