Why do teachers always bring up "I'm an adult and you have to listen to me"?

Okay, so, I know that we're supposed to listen to people of higher authority, but I mean they always bring it up. It kind of annoys me, I lose respect for them when they bring that up every single day. Like my math teacher, sometimes when we're just doing homework for the last 30 minutes of cl, someone gets up to sharpen their pencil and the teacher says "Sit down". When we try telling her that we need to sharpen the pencil she always says "No, I'm an adult, so you have to listen to me." That bugs me big time, I feel like we have no say in things whenever they say that.