Address the teacher by name. Be formal about this, using "Ms. Clarke" rather than "Angela." If you are sending the same letter to multiple teachers, you should create separate letters with each teacher's name, rather than a general "To Jill's Teachers."
Write a brief statement that you want to introduce your child, including her name. This shows the teacher the reason for your letter.
Start with your child's likes and positive traits. Mention her love for books, for instance, or her excitement about coming to class.
Detail any problems that your child might have. This might be a medical problem, such as ADHD, in which case you might want to talk about any medications that he's on. Alternatively, it could be a family situation, such as a divorce or death in the family, that might affect your child in class.
Offer tips to help your child's teacher deal with negative situations. Giving solutions to potential issues can help the teacher avoid major problems that could disrupt the whole class. Mention that your child responds much better to praise than to negative criticism or that she can focus better if she sits in the front of the classroom away from the window. A small thing like this could make all the difference.