Despite an intensive registration effort over the summer, Washington, D.C., public school enrollment has dropped more than 8 percent since last year, the steepest annual decline since the District first hired an outside auditor to verify the student population in 1999. Three weeks into the fall term, 45,135 students were enrolled in the District's 120 schools. That is down from 49,422 during the 2007-08 school year. The drop comes as the popularity of the city's public charter schools continues to grow. Although charter officials will not undertake a count until early next month, they are projecting a 20 percent gain, to 26,494.