Nicole Johnson stood behind a marimba in the middle of a Somis classroom, her arms frantically moving up and down as her mallets struck the wooden keys.

"Keep those arms going," her music teacher, Scott Murphy, called out.

Nicole groaned as she kept up the frenzied pace, a smile never leaving her face. A few beats later, other students joined in and a melody of Zimbabwean xylophones filled the classroom and flooded the empty campus of Somis School.

The last bell on Friday afternoon had already rung, but the 10 students inside Murphy's classroom showed no desire to leave and start their weekend.

They are part of a growing after-school marimba program at Somis School, one of the only, if not the only, public school offering such a class in Southern California, Murphy said.

It started a few years ago after Murphy got hooked on the African instrument at a university class. But it was a year from idea to students playing the first note. They had to build them first.

Nicole was a bit skeptical when Murphy first recruited some of his band students to build their own marimbas. The 13-year-old and several other bandmates learned to use a chisel and a small hammer to peel back layers of wood until each "key" plays the right note.

At first, Nicole said, she didn't think it would work. When it did, "I was amazed. ... You're banging on a piece of wood."

Murphy started a marimba class as an elective this year, allowing students to participate even if they can't stay after school. He also has continued the after-school program, with students building for an hour Friday afternoon and then setting up the already-completed marimbas to practice.

Murphy's goal is to raise enough money through performances, grants or donations to buy a set of seven professional marimbas for his students to use in school. Then, they could take their handmade instruments home to practice.

Students said they would welcome the extra homework.

"I love the marimba," said Elan Ehrhardt, who flings his arms up and down as he plays. The music, he said, "it's kind of just happy."

Elan and a few other students moved to new schools this year, but they continue coming back to Somis for the after-school program.

It's hard work, Nicole said, but "everyone here is having fun."

Mary McKee, superintendent of the one-school Somis Union School District, tapped her feet to the beat inside Murphy's classroom Friday.

She remembers being a little nervous about students building their own marimbas. She signed off on the program, trusting in Murphy's ability and focusing on the importance of music and hands-on learning at the school.

She's glad she did, McKee said Friday as she looked around the room at the students singing and bouncing mallets off the homemade keys. Murphy strolled around the room, keeping a steady beat with a pair of maracas.

"It's not often you see kids at school at 4 o'clock on Friday," McKee said, smiling.