Scores of teenagers streamed out of John McDonogh Senior High School in this city's storied Treme neighborhood on a recent afternoon as a dozen security guards and city police officers stood watch along the sidewalk. The three-story brick school sits on Esplanade Avenue in Treme, the neighborhood just north of the French Quarter that is famous for producing some of the city's most talented musicians and brass bands. In January, a shipment of new trumpets, saxophones, and other instruments made it possible for McDonogh to reassemble a 130member marching band in time to rehearse and polish a routine for last month's Mardi Gras parades.