Art can be a difficult subject to teach because there's not technically a "right" or "wrong" way to do it. A lazy student can show up to art class, put no effort into their work, and claim that their low-effort piece came out exactly how they intended it to. That's the oldest trick and the book, and any teacher should be able to spot it. There are ways to instruct and critique students' art throughout their process of making something that can help them achieve the aesthetic bliss they're going for. Teachers can ask students questions about the choices they are making and offer advice that points them more in the direction of their goals.
There are some pedagogical methods that art teachers should avoid at all costs. I'd say the number one no-no is tampering with a student's art without their permission, especially when a student is an older high schooler who can listen to directions without it being done for them. An art student recently went viral on Twitter after her teacher tampered with her sculpture without her permission, and the before and after are utterly baffling.