Simply replace the unfilled areas such as "" and "[date]" with your information.
Dear [name of teacher],
Thank you for your [letter/phone call] of [date of contact] regarding [name of child]. I appreciate the time you took to voice your concern, and wanted to follow up with you regarding this issue.
When you made me aware of [issue], I had a discussion with [name of child] regarding the problem. We have made some plans and decisions about how we are going to resolve this issue, and I feel like you will begin to see improvement in this area. I have made [name of child] aware of my expectations in this area, and [he/she] understands that you and I will be communicating more on this issue.
Thank you again for your assistance with this. Please let me know if the issue does not improve.