Just curious why did this interaction make you mad? If an painting student does nothing but paints eyes, that student will become really good at painting eyes. If a music student plays one song, they will get really good at playing one song. I've been in acting classes where teachers have had similar reactions, telling me and other students to not treat a scene like this or that. I took no offense to it personally and found that it helped me in the long run. That is not to say that the teacher couldn't have taken a moment and said "That was very good, it was good because you knew it so well, so lets not do that again" but I do not see the purpose in getting angry at such a phrase (except of course in the history of the Julliard program that Jacobs lays out here).
Epistemological status: personal and shaky Based solely on my experiences in the school system, my experiences in unschooling/homeschooling, and the experiences of homeschooling parents and children who I've talked with, I'd say that creating a positive environment focused on giving students tools they need is significantly more important than cutting them down when they succeed to guide them, in all circumstances. Shaming, dismissing, insulting, and denigrating effort are all recipes for students to shut down, to stop experimenting, and to withdraw. For an example, the above article: Jacobs plainly says how such comments hurt her deeply and damaged her ability to perform. Her ability to cope by adopting a strongly different personality is not healthy, nor is it conducive to learning. I became angry because I've been in those kinds of situations (and the others described, such as when a teacher complimented her by saying, "Wasn't probation good for you?"), and they're awful. I've never felt so terrible as when I've seen my honest efforts dismissed and disregarded by someone who thought they knew what was best for me. I would never wish that on anyone, and that you say you weren't affected by it when it happened to you probably means that the effort you put in wasn't meaningful to you the way this comedy routine was meaningful to Jacobs. Because it's not actually about acting, it's about effort and reward and support. If the teacher wanted to say, "You can clearly do that. Never bring in a scene like that again." but kindly, like the listener deserved respect and support instead of dismissal and shame, they could have said, "That's wonderful! Well done! You seem to have found your niche! In the future, let's see if we can work a scene like this into your portfolio while we work on your other styles. Obviously you have talent, but can we bring that talent to other genres?" and then when she tries (and maybe/probably fails) a different genre, the teacher can bring in references to the progress she's made relative to her comedy, instead of demanding from a non-existent place.