WOODBRIDGE, Va. — Walking the halls of Cecil D. Hylton High School outside Washington, it is hard to detect any trace of the divisions that once seemed fixtures in American society. Two girls, a Muslim in a headscarf and a strawberry blonde in tight jeans, stroll arm in arm. A Hispanic boy wearing a Barack Obama T-shirt gives a high-five to a black student with glasses and an Afro. The lanky homec