National public education statistics put the ratio at 51.2% girls as of 2006, the latest year available. Previous years show similar numbers.

http://nces.ed.gov/programs/digest/d08/tables/dt08_053.asp

My DD's anecdata backs the official numbers, as she has always reported her GT classes to skew more heavily to girls. Her district uses a formula that combines RIAS, achievement scores (dunno which test), and teacher recommendations.