"We're gonna dance for the last time!"

During rehearsal, Palo Alto Chamber Orchestra music director Ben Simon gives his students a suggestion to make their final rendition of a Bach piece more spirited. Students start to sway from left to right as they play their instruments. Bows graze across the strings in the vivacious Brandenburg Concerto No. 3 in G Major. All the musicians dance through Bach's work -- even the seated cellists.

Many students are busily finishing the last bit of homework and preparing for the new week on a Sunday night. For those in the senior ensemble at PACO, though, this evening is reserved for weekly practice at Palo Alto's Cubberley Community Center.

"Music isn't for oneself completely, but the power of music is to share and play for others," Simon says. For its 45th birthday, PACO is sharing by holding a benefit concert on March 27 at Costano Elementary School in East Palo Alto. Proceeds will go to the Music in the Schools Foundation, a nonprofit that provides music education to the Ravenswood City School District.

The beneficiary will also be a part of the concert. Thirty student choral singers in the third through fifth grade from Green Oaks Academy in the Ravenswood district will be sharing the stage with senior members of PACO.

The concert also features a folkloric South American trio called Chaskinakuy, based in Northern California. The group plays Andean music with costumes and instruments made from a wide variety of materials. Among these is a violin formed out of an armadillo shell. Simon said he invited the trio to add variety to the concert program because of their easily accessible sound.

All five ensembles of PACO will perform short pieces. At the end of the concert, more than 150 current and past PACO members are scheduled to perform the Brandenburg Concerto No. 3 in G Major. To reconnect with alumni and celebrate the occasion, Simon invited several alumni and guests to play alongside current musicians.

The concert will be collaborative on other levels as well. The senior ensemble will provide background strings for the Chaskinakuy trio and for the students at Green Oaks, with both string arrangements by Simon.

"An intense musical soup" is one way Simon describes PACO. The orchestra is distinctly chamber-based. Simon is intent on keeping the orchestra small -- there are about 20 to 25 students in each ensemble.

Despite PACO's size, its commitment to strong chamber music has allowed the orchestra to grow in the number of ensembles, scope of repertoire, touring and recording, Simon said. "But the heart of PACO remains the same," he added.

Camaraderie and concentration marked the mood at the rehearsal on a recent Sunday evening. Students seemed excited to catch up on each other's weekend during down time, but focus characterized the group when practice began.

Friends Isabella Costanza and Joanna Chang stood only a few music stands apart. Costanza has been playing the violin for eight years and Chang for 11 years. They also both play the piano.

"We don't judge each other -- we support!" said Costanza, a freshman from Gunn. She said she doesn't mind practicing two to three hours of violin a day; she wants to pursue music professionally one day. Numerous hours of music have "become a part of us," she said.

The group of musicians, as Simon said, is connected by a musical and interpersonal bond. Everything in PACO is a shared experience, from weekly rehearsals to international tours.

As Costanza and Chang chimed in on each other's conversations as close friends, Chang fondly brought up an orchestra tour last summer.

"We went on tour to Turkey and had a lot of fun," she said, recalling that PACO played her favorite piece, "Las Cuatro Estaciones Portenas," by Astor Piazzolla. Its structure is "kind of like Vivaldi's Four Seasons," she said.

PACO is divided into five orchestras, starting at Superstrings, with musicians then moving up to the Preparatory, Debut, Sinfonia and Senior levels. But for Chang and Costanza, both members of the senior ensemble, there is no disengagement with one another based on level or age.

"It feels like we're a family," Chang said.

Simon wants his students to perform not only as an orchestra but also as a community, connecting musically within the orchestra and working with other students and organizations.

Virginia Fruchterman, the chair of Music in the Schools, said in a phone interview that "people seem to notice that music is disappearing (in school)." She added, "Fundraising is a little difficult but it's a little up this year."

The program currently offers classes that teach the basics of music and rhythm through musical games, singing and dancing. Older students are learning to play the recorder. The district is not charged for the classes, according to Music in the Schools' website.

Fruchterman said she hopes to also start a strings program and add more after-school music programs. Music in the Schools currently reaches 1,000 students a week, but this is only a third of students in the Ravenswood district, she said.

"There are more who want music," Fruchterman said.

What: The Palo Alto Chamber Orchestra marks its 45th anniversary with a benefit concert for the Music in the Schools Foundation.

When: Sunday, March 27, at 3 p.m.

Where: Costano Elementary School, 2695 Fordham St., East Palo Alto.

Cost: Tickets are $25 for adults and free for children under 12.

Info: Go to pacomusic.org or call 650-856-3848.