The prelude is off to a rocky start. It’s the first song of the evening rehearsal, and the students have barely warmed up. Some notes are off beat; someone’s out of tune; the conductor’s motions are verging on frantic.

But then something happens. Everyone finds the rhythm, some 30 instruments in harmony with each other, bows rising and falling in sync while the timpani enters the crescendo and the music swells to a breaking point. Then with one last crash of cymbals, everyone falls silent.

This is why they’re here.

“That’s it!” conductor Bruce Walker exclaims, brandishing an ecstatic thumbs-up. “That was just so professional.”

The Yakima Youth Symphony Orchestra has been around since 1965 and this year includes 40 students from all over the Valley, who come seeking a musical outlet unlike any they can find in school. The symphony gives them a chance to play with band and orchestra combined, and to tackle music beyond what they usually encounter in class.

“Performing — there’s no words to describe it. It’s just this beauty that you can only get when you’re playing with everybody,” said Hannah Erickson, a junior at Selah High School. This is her first year playing flute with the symphony, which practices Monday nights at Englewood Christian Church. This week was the last practice before their Sunday concert in the Capitol Theatre.

The group is mainly high school students, though admittance is based on skill rather than age. For younger or less experienced students, there’s the Youth Ensemble for Strings, which Walker calls a “training ground” for the symphony. The two programs together are open to musicians ages 8 to 21, and have 76 students this year.

Both are now under the umbrella of the Yakima Symphony Orchestra, a registered nonprofit that has brought adult musicians in the area together since its incorporation in 1971. But the youth programs manage their own fundraisers, concerts and rehearsals, with Walker and a parent governing committee in the lead.

This is Walker’s second year as conductor. In the youth symphony, he conducts with his whole body, furiously waving his arms and using big facial expressions to tell his musicians what he wants them to do. But while they might joke around, he says, they’re serious when they sit down to play.

“There’s a different level of maturity that’s in place and a different level of expectation being put on students,” said Walker, who also teaches music at Sunnyside High School, conducts for a preparatory orchestra in Pendleton and serves as cover conductor for the Yakima symphony, an understudy-type role.

He’d like to see the youth symphony double to 80 students, and they’re holding auditions for spring semester on Jan. 5. Students must audition to get into the youth symphony to prove they can handle the music he assigns.

“I do have high standards; I expect my students to achieve a certain level. However, the easiest way to get there is when you’re relaxed and you have the flexibility to experiment ... so you’re not afraid to make a mistake,” Walker said.

His methods seem to be working, as his students say they love rising to the challenge.

“(Practice) doesn’t feel like it’s two hours,” said first cellist Haley Rockholt, 17, who attends Davis High School. Walker has been her cello teacher since fifth grade.

At school, she said, “There isn’t an opportunity for the two groups (band and orchestra) to mix at all and play the music that’s written,” rather than an arrangement. “You don’t get to hear all of the different brass, woodwinds, percussion ... and if you’re in the band, you don’t get to hear the strings.”

With symphony, she gets to hear all those instruments and meet the young adults who play them.

But Walker is quick to reassure music teachers.

“The youth symphony orchestra is not a replacement for any program that’s in the school. This is only to enhance those particular programs,” he said. “I’m a music educator ... we get very territorial of our students.”

But for students who want to take their learning to another level, symphony provides that opportunity.

“It’s a totally different group of people. Sometimes it feels like people are in band just because, but here, it’s because they want to, and they love playing music,” Erickson said. “People are actually practicing their music.”

There’s a financial incentive to be committed to the group: It costs $400 a year to join the youth symphony, and $300 a year to be in the youth ensemble, though the programs don’t want money to be a barrier to anyone and provide financial assistance when necessary.

Tuition goes toward the conductor’s pay, sheet music and other program production costs.

First violinist Catherine Fasset, 17, practices two hours a day and has played for six years. She appreciates the dedication of her peers.

“You’re all serious about something and you have something in common. ... It’s a fun group,” she said.

Fassett is home-schooled, so she also enjoys making friends through the program. This is her second year in symphony; prior to that, she had only done solo music.

“It’s a totally different experience,” she said. “It’s like this waterfall of sound and you’re one drop in it. It’s really amazing.”

The Yakima Youth Symphony Orchestra is holding its winter concert at 3 p.m. Sunday at the Capitol Theatre. Admission is free, though donations are accepted.