But step inside and you may hear the opening notes of Bach’s Brandenburg Concerto No. 3 spilling from a second-floor rehearsal room filled with 15- and 16-year-old string players. Or youthful fingers plucking a traditional Venezuelan folk tune on a harp, accompanied by a soft percussive rattle.

In the poor hillside neighborhood of Chapellin and at nearly 250 other locales throughout this nation, tens of thousands of young Venezuelans are learning to play classical music and to make art a permanent cornerstone of their lives. They’re the latest recruits of El Sistema, or the System, a 33-year-old program that many regard as a model not only for music instruction but for helping children develop into productive, responsible citizens.

Without doubt, El Sistema’s most illustrious graduate is Gustavo Dudamel, the 27-year-old conductor who next September will take over as music director of the Los Angeles Philharmonic. Yet El Sistema measures its success less by how many world-class professional musicians it churns out than by how well its pupils are able to absorb the values of self-discipline and teamwork in service of social harmony.

Roughly 80 percent of the students are from low-income families. Some have been afflicted by domestic violence, parental drug addiction or worse. Many live in urban barrios such as San Agustin, a sprawling development in central Caracas. Others have serious learning and/or physical disabilities. El Sistema takes all comers and gives them the same chance — including free instruments. The Venezuelan government contributes $29 million annually.

Mayra Rivero, a teenager who started out singing in an El Sistema choir then moved on to playing the cello, says the program has affected her life in many ways, all positive. “I’ve improved, for example, on an academic level, because here we study a little history about the music,” she says, “and on a personal level also, my friends above anything else.”

El Sistema is the brainchild and lifelong spiritual mission of Jose Antonio Abreu, 69, an economist and politician. When he started the program in 1975, with 11 children and a handful of volunteers operating out of a garage, a few skeptics scoffed at the idea of imparting classical music to the disadvantaged. Today, about half a million children are enrolled in El Sistema’s training centers, called nucleos, and the program has been copied throughout Latin America and in Europe.

“Art education is an essential component of the educational system,” says Abreu, a deceptively soft-spoken man with the fiery social conscience of a Jesuit reformer, speaking at El Sistema’s central offices here. “It cannot be a peripheral element. It’s not possible that a child would have access to an arts education as an option, by accident or out of charity. Because an aesthetic formation is that which touches our sensibility. Art and religion influence, definitely, the formation of our values.”

Nucleos are swarming hives of cooperative activity. At San Agustin, students ages 7 to 18 wander purposefully between rehearsal rooms, noodling on trumpets and woodwinds, or sit thoughtfully absorbing stacks of music. In one classroom, a group of 7-, 8- and 9-year-olds loosens up with breathing exercises. In another, an older-student orchestra rehearses Arturo Marquez’s “Conga del Fuego Nuevo,” a standard in El Sistema’s repertory.

Older students, as they advance, eventually train younger ones, enhancing the feeling of being part of a surrogate extended family.

At the large nucleo in Dudamel’s native Barquisimeto, many students have disabilities. Children with Down syndrome, who tend to have small hands, are given tambourines and larger percussion instruments to tap. Hearing-impaired students perform sign-language interpretations of choral numbers alongside their hearing colleagues. A blind guitarist strums and croons a traditional bolero. It is routine in Barquisimeto to see a student in a wheelchair receive a helpful push up a ramp, or a deaf student patiently hold the door for a blind colleague.

Among El Sistema’s admirers is the Los Angeles Philharmonic, which has launched its own pilot program, Youth Orchestra L.A., with 150 students, in an effort to provide music instruction to children in underserved areas of Southern California.

“It would be great to export El Sistema to even more countries,” says Alexandra Pineda, 34, a music teacher at the Chapellin nucleo. “It would be excellent to have a little piece of Venezuela in all the world.”