A person who feels the need to instruct others how to perform better at something, despite lacking the background to do so or the ability to translate their knowledge effectively so people actually learn something. A clown teacher is often unaware of how much they are unappreciated and/or disliked.
Mark: "Mr. Gobs failed me again. I pay attention in class but I guess he's not teaching us well enough."
Ryan: "Don't worry about him man. He's a clown teacher. Nobody learns anything from him."