Have a fun day rehearsing and singing a major piece of music in concert under the guidance of a professional conductor. At the same time, raise money for a good cause. That was the idea five years ago, in the first Come and Sing concert for Oxford charity The Art Room.

The idea clicked, and four further Come and Sings have followed, raising around £30,000 for The Art Room, set up in Oxford in 2002 to offer art as therapy to children and young people aged five to 15 who are having difficulties. Art is used to raise their self-esteem, confidence and independence, and help them cope with their challenges. There are four Art Rooms, including one at the new Oxford Spires Academy. The others are at Rose Hill Primary School, Orchard Meadow Primary School, and Robert Blair School in Islington, London. The charity has branched out and run family workshops as well. An outreach service is being piloted at Cheney High School.

Now singers are being assembled for the sixth Art Room Come and Sing, which will feature Rossini’s Petite Messe Solennelle. Written more than 30 years after the last of his much-loved operas, Rossini originally conceived the Messe as a small-scale, intimate piece. But he saw opportunities for expansion.

“Rossini decided before he died that he might as well orchestrate the work as he wanted it, otherwise someone else would do it after his death,” Come and Sing conductor Robert Dean explained. “What’s funny is that Rossini specifies a huge orchestra, vast: he really wanted to up the ante as far as the scale of the piece was concerned. Looking at the score, I think he felt: ‘in for a penny, in for a pound’. So I think he would have approved of us having around 250 people in the chorus, in a piece originally written for 12.”

All of which means that Robert will have his hands full on the day. What are likely to be the hairiest moments?

“Keeping the fugues together. Simply because of the layout of the Sheldonian, you’re trying to wave arms in all directions, and hopefully things will meet in the middle. It amazes me how it does all come together. Often the rehearsal is very rocky, but when we come to the performance everybody is concentrating like mad because it’s a do or die scenario.

“As the conductor, you’re learning all the time. I’m finding now that I have to make every gesture mean something, because I have very little time to explain things in words. As for the event itself, dare I say it, very little actually goes wrong during the course of the performance, because everyone is concentrating so hard. It’s very invigorating. Last year everyone was saying: ‘You can’t do Mendelssohn’s Elijah in a day’. But we did. It was pretty good actually, I was quite proud of it!”

Besides a large choir and orchestra, the Petite Messe Solennelle requires four soloists.

“The tenor solo is quite renowned in this piece, and has been recorded over the years by many famous tenors,” Come and Sing soloist Nicky Spence explained. “However, I can only sing it my way with my voice, so I'm fighting the temptation to refer to the greats for inspiration before I've decided how I'd like to portray it.”

Nicky was also a soloist in Elijah. Self-styled on his extrovert website as the “Voice of Scotland”, he lists tours with likes of Bryn Terfel, Katherine Jenkins, and Hayley Westenra amongst his accomplishments. What was his reaction when first invited to appear at a Come and Sing concert?

“I was thrilled to be asked, as it’s for such an honourable cause, and it was great fun a year ago. It’s also nice to be re-booked to do something as it means you didn’t offend too many folk the previous time!”

Petite Messe Solennelle is at the Sheldonian Theatre on February 12. Audience tickets: 01865 305305 or www.oxfordplayhouse.com/tickets oxford Tenors and basses are still required, e-mail for details: comeandsing2011@hotmail.co.uk