"You have really made a difference this year for [my son]. Before, he was under the impression that math was not his thing and you have been able to make him see that he is capable. As a result he has become much more confident about his math skills! Thank you!"
"This is [so-and-so's] mother and I am writing you because I am especially grateful for your work. I must mention that the time we started at [your school] was very difficult because the school doesn’t have math levels and we felt that his math lessons were lacking in engaging, intereactive and varied, [my son] felt he could give more. So [this year] started with a lot of expectation about Math curriculum and how the school could improve it in order to leverage the children talents not only for our son’s benefit. And I think the school did it because of you. I saw [my son] enjoy Math again and I want to thank you for making my sons math lessons his favorite."
Remind me when I become a parent and I get on the other side, that the best way to help a teacher feel motivated to do a better job is with positive reinforcement!