indeed the boy violated the rule ,but not publically. with out making any noise he went out.so others did not know that the boy left the room. the boy did not encourage others to commit mistake. so he was given a lesser punishment, instead of getting burnt to ashes he became only brahmarakshase. the man who removed the curtain was doing the mistake infront of others ,he was misleading others also even after getting such a wisdome from the guru. that is why he was burnt to ashes. why the remaining people also got punished ? because they did not prevent the man from violating the words of guru . they silently allow the man to remove the curtain. so they were also participated in the crime even though they were sitting idle.