I always shudder when I hear that question
Every lesson students ask me those three words. Even though I have taught them better it seems instinctual. The danger of answering is that we have just allowed them to submit their art to the value judgement of someone else. If they are expressing their thoughts or ideas or emotion through their work then by asking "Is it good?" they are undoing every risk and experiment that they just participated in. I wonder if Picasso or Pollock asked "is it good" to their peers?
So, instead of answering their question I engage in a learning conversation... It turns the question on them, and helps them to either properly frame their question "Have I used the process correctly?" or discuss what they are communicating; for example...
Student - "Is it Good?"
Teacher - "What do you think?"
Student - "I like it."
Teacher - "What do you like about it?"
Student - "I used several layers of colour... I chose red and yellow to show anxiety, and then went over with vibrant blue to show courage, but then splattered red to show fear rising, but scratched that off to express walking through that fear" (an actual conversation with an 11 year old)
Im sure you all do this in some way, but in the busyness of teaching art, it can be so easy to just say 'yes' or let students slip into habits of looking for 'good' over looking for creativity, expression and ingenuity.