As a former teacher myself, I am disappointed and surprised that she had no knowledge that he was left-handed; if she is demanding he use his right hand for writing and coloring, she should stop doing so and let him express himself naturally, which is left-handed. If you continue to observe fine motor dexterity at home and can enter into a reasonable discussion with his teacher about not making him feel self-conscious about his coloring and writing, then you will have done your part. What is most important at this stage regarding school is that he develop a curious, healthy, excited attitude about learning and that he is appreciated by you and his teachers for exactly who he is and what he can do at this time. Good luck.
Motor Skills Problem in Five-Year-Old
Toddler and Teenager Expert Advice from Carleton Kendrick, Ed.M., LCSW