O'Neil Hemmings, headteacher at St Saviour's primary in Lambeth, south London, said he noticed a dramatic improvement among five to 11-year-olds who took up the instrument.
He said: "We've noticed that behaviour has improved among pupils across the school who learn an instrument like the violin, which is very difficult to master.
"One Year 6 boy was always getting into fights and arguments, and back-chatting the teachers. Once he started learning the violin his self-esteem shot up. Over time we noticed he wasn't getting involved in trouble anymore.
"I've seen that if they are disciplined about one thing there's a ripple effect. Their whole perception of learning changes because they are not perceived as the naughty person, but the one who plays the violin well."
Gill Morley has been teaching the violin at St Saviour's for two years to nine children, aged between seven and 11. She said: "If they feel angry, they can play angry music and get it out. If they feel sad, they can play sad music."
The Nordoff-Robbins Music Therapy Centre in north London works with pupils from local schools, including Gospel Oak primary. Helen Patey, the assistant director, said: "The pupils we work with are just finding life a bit difficult at the moment. For some children the violin could be too difficult. We work with drums, or xylophones, instruments which anyone can play."
Dr Jane Davidson, former editor of Psychology of Music, said: "It's about creating a forum where the children are all equal starters. I've seen disruptive kids who didn't want to concentrate get hooked on music, whether that's rock and roll or Baroque recorder."