What started as a rehearsal at the Kennedy Center for a student choir from the Duke Ellington School of the Arts turned into a mini-John Legend concert when the songwriter dropped by to surprise the group.

View Photo Gallery: Singer-songwriter John Legend and the Kennedy Center announced the launch of a digital youth campaign, “What’s Going On . . . NOW,” dedicated to encouraging students to get involved in the arts.

The students, practicing at the Kennedy Center for their Tuesday night performance at the Millennium Stage, had no idea the Grammy winning singer (“with a voice like molten lava,” one visitor swooned) would be swinging by their rehearsal.

Legend was at the Kennedy Center Tuesday morning to participate in the launch of the Center’s “What’s Going On…Now” project meant to both inspire young people to engage in their communities through the arts and to commemorate the 40th anniversary of Marvin Gaye’s “What’s Going On.”

“Can I play with you?” Legend, a longtime Gaye fan, asked the totally dumbstruck students, before sliding onto the piano bench and joining in their rendition of “What’s Going On.”

“The Kennedy Center presented this idea and I couldn’t imagine saying no,” said Legend later. “This album has been really influential to me. And to do it here, in D.C., where [Gaye] is from, that’s quite humbling.”’

The national arts and digital media initiative will reside at WhatsGoingOnNow.org, where teenagers can upload art of their own — music, video, poems, and the like — and ruminate on how the issues expressed in Gaye’s song still feel relevant in 2012.

Legend will perform at two concerts at the Kennedy Center on May 3 and 4, along with the National Symphony Orchestra, Sharon Jones and The Dap-Kings. Legend was on hand at the kickoff today

The “What’s Going On… NOW” project, he said, “encourages [the students] to know more about their musical and cultural history, so they understand the legacy that they are living.”

Legend’s role in the project between now and the concert is mostly as an awareness ambassador, spreading Gaye’s gospel to students across the country. “Not only are we reviving this beautiful piece of art,” he said. “But we’re making it alive for young people and hopefully giving them a message for their community. “