Classroom of Truth is a manga written by Toru Nanamine. It was submitted for the Treasure Award, but didn't win due to not having any Shonen feel. After that, he published the manga on the internet.
One day while Takashi is in class some sort of tremor occurs. After a voice tells them they are in the Classroom of Truth and doing a certain thing will cause them to die. They soon realize that if they lie they will die so after the first three deaths they decide not to say anything so the voice gives them Q & A. If they don't answer in 10 seconds they die. When the voice asks who stole a certain girl's gym clothes the culprit panics. Instead of telling the truth he lies and says he is 14 not 13 but the voice tells him that if you lie because you are trying to die on purpose then you will not die. After the Q & A, the voice tells them to pair up but because there is an odd number of students the one by themself will die. When one kid asks his friend to pair up his friend denies claiming the friend is weak and he needed someone strong because they would have to fight against each other to survive. Because he says this they tie him up and nobody pairs up with him, resulting in his death. When round 1 ends there are only 8 students left and they all die. The main character dies in round 3 when eaten by his doppelganger.