The Bradwicks’ elaborate Park Slope brownstone was bursting with teenagers when Sierra and Bennie got there. Just about every ninth, tenth, and eleventh grader from Octavia Butler High was running around the backyard or exploring the winding passageways of the house. The sound system alternately blared hip-hop and grungy emo rock as various DJs took turns pushing one another out of the way. Some kids stood in a little circle out back, beatboxing and freestyling, inventing brand-new ways of putting one another down and sending up wild cheers when each dig found its mark.