Kin'iro no Corda

Kahoko Hino is a student at a Seisou Academy, one with an ordinary life. One day, Lili - the mischievous musical fairy who had blessed the original founder of the school - finds Kahoko running late to class. Delighted that she is able to see him, Lili grants Kahoko a magical violin, and a place in the school's annual musical competition, which many students in the music department vie to partake in.

Kahoko refuses, only to be pressed on by the fairy, who constantly nags her until she finally gives way to his pleads. She reluctantly accepts the instrument. As she practices her pieces, Kahoko is amazed that she can play any piece on the violin as long as she knows the tune and plays it with her heart.

[From Wikipedia]