Please imagine that you take a friend (a guy) to meet a group of your friends at a party. The group and the guy are have never met one another and know nothing about one another. The guy starts clowning around out of the blue and you have no idea why he is acting like that. You feel shy in front of your group of friends and wish to let the guy know that you feel uncomfortable with his actions. You wish to tell him to not act foolishly and present the appearance of a low-class guy in front of your friends. In my mother language we may use any of the sentences below. I don't know if there are some equivalents in AmE to convey similar messages, or if they all work in English. I would appreciate it if someone could let me know if there is a natural sentence from among my suggested examples I could use, or if not please tell me what an AmE speaker would say instead:
Be like a high-class person.
(Observe / maintain) your class.
Don’t act like a low-class person.