Piano no Mori

From Easy Going Scans:
A tranquil tale about Shuhei Amamiya, who transfers to Moriwaki Elementary filled with hope and ambition about his new life. But it doesn't take long before he gets picked on by the class bullies, and gets involved in a dare to play the mysterious piano in the forest, leading to his meeting an enigmatic child that goes by the name of Kai Ichinose, who seems to be the only one capable of getting sound out of the thought-to-be broken piano. His ability earns him the respect of Shuhei and his music teacher, former master pianist Sosuke Ajino. At first Kai is resistant to refining his art but after hearing Sosuke play a Chopin piece he just can't seem to play by himself, he relents.

Note: Received the Grand Prize for best manga at the 12th Japan Media Arts Festival in 2008.