Since, like most Graphic Design teachers, I started teaching at the end of a professional career, I wasn't prepared for how completely awful the art and design of students could be. Even the very best of the best was so bad that it wouldn't have gotten a second glance in the professional world. So I had to learn very quickly what the standard of student work was, and grade and criticize accordingly.
Fingernails scratching along a chalkboard pretty much sums up what looking at bad art and design feels like to me. But it was part of the job, so I did it. And since I wanted to encourage, not discourage, I took a different angle on this than what most art teachers did.
Mostly, I smiled and said something positive. When someone showed me something that looked not unlike something that a designer might have done on a bad day, in the dark, with their eyes shut, I would say, "this is a good start!" For many people, this was pretty harsh criticism, and they would quietly slink back to their desks with their perfect masterpiece.
My better students, of course, couldn't be fooled. Since I was always so overjoyed to see ANYTHING of any worth at all, it was impossible for me to hide my happiness behind a "poker face". And since my goal was to encourage, not discourage, I was quick to ask if I could show the work as a great example. I could usually count on one or two people in a class of about thirty who could show that something correct, and visually appealing, could be done.
Yes, 99% of the students say that the teachers are "so mean". But the one percent is someone who once said to me "stop just giving me As, tell me how to make my artwork better". I can still here the voice of this student, and hopefully he will drown out all of the other voices in my memory.
This is a good start!