Simply replace the unfilled areas such as "" and "[date]" with your information.
I am writing this letter to express a concern with [Child]'s recent grade on the [specific test/homework]. I regularly review all of [his/her] work and scores. Upon review of this [test/homework], it has come to my attention that there may have been an error during the grading process.
I believe the answer to Question #[number] is actually correct, although it was marked incorrect. I have researched this question and verified that [Child]'s answer is accurate. My supporting information is:
[written solution to problem OR documentation & source]
I realize this may have been an oversight. I know that your dedication to your students keeps you very busy. I appreciate you taking the time to correct this grade. If you disagree with this answer, please contact me immediately so that we may resolve this issue. You may contact me at [contact information].