The Lost (song)

"The Lost" was a folk song once performed publicly by April MacLean, as her mother cheered her on. She played the violin to her own vocals. (TV: The Lost)

Behind the scenes
The track was co-written by Jim Moray, Blair Mowat and Patrick Ness, and followed "Nightvisitor" as the second original song on Class. "The Lost" was the original song first to feature diegetically.

Sophie Hopkins indeed recorded the song, which can be heard on Blair Mowat's SoundCloud account.