They wouldn't let you leave until you admit something? So you're telling me that if you stood up and walked out of the door they would have grabbed you and forced you to stay? They cannot legally force you to stay there until you incriminate yourself. You fucked up by saying you smoke weed, you did not have to admit to anything.
Not true. In high school they can tell your parents anything they want no matter how old you are. You lose almost all your rights when you walk into a high school.