I'm still a little confused...most people send thank you letters at the end of a school year. How old is your child/what grade? Have you heard good things about this teacher from other students? Are you trying to send something at the start of this year to tell them about your child's specific interests or learning style? I have written letters to my child's teachers at the start of the new school years in order to inform them about my child's strengths/weaknesses, but other than saying "My child (name) and I are looking forward to a wonderful school year. Thank you in advance for helping my child to reach his potential" it is rather difficult to give a note of specific thanks.