For people who doesn't get it. (It's what i think, also i suck at english).
The boys we see sing and the thing we see comming out from they mouth (bubble, flower, Bunny?, etc) represent how beautiful their singings. But when the boy in the middle sing, insect came out. It show that he's not really good at singing. There's also an other boy with short hair that show up with the teacher singing so good that starry Sky and star came out. People are amazed by how beautiful his sing is. The mid-long hair boy feel belittle or inferior. Mid-long hair boy exerce his sing alone, feeling lonely. The short hair boy appear sing along and so mid-long hair got jalouse and choke him to death. That was actually his thought, he didn't actually choke him.
