That teacher was a jerk! It was wrong of him to say that to your son, and you should complain to the principal. But if your son seems dead-set on giving you the "perfect" flower, you can offer him a compromise. Tell him that he can give you whatever flower he has, because that's what he made for Mother's Day. However, also give him an opportunity to "make it up to you" by making you a nicer-looking flower another day. This way, you can still praise him for his work, and he'll have a chance to make something he thinks is better.