Class off

Today in our school Bengali sir was absent and students in his class were making too much noises and involving themselves into funny business as he was absent so there was no one to alter him. So I walked in the class and said -

Students it is your class off because your bengali sir is absent today, so it doesn't mean you won't keep quiet and keep involving yourself into the funny business.

Am I grammatically correct here is the "class off" fits here to describe the situation.