STOP. Don't allow yourself to get pulled into an E-mail chain battle with a parent (or anyone else)
If you and a parent have a disagreement and cannot work it out in 2 messages. STOP. Either call and talk, or ask your learning director to mediate a personal meeting. Usually a call from the LD stops the problem. It also provides a new perspective. Maybe your 100% right! Maybe your 20% wrong!
I don't like long E-mail discussions. They take a ton of time and people are quicker to resolution, and nicer, when you talk.
Hope this helps,
Love to Teach and Teach with Passion
Remember...It's not Magic, It's Science