Hello there;
It sounds to me like your head teacher just wants to make sure this is something you've thought out carefully, and discussed with your parents. It's going to be really important that you sounds calm and confident when you're writing the letter, so he knows you're making an informed decision.
In your first paragraph you need to start by introducing why you're writing to him, something along the lines of:
"I'm writing to you to explain my reasons for wishing to drop History, as we previously discussed"
And then go on to outline how you're feeling.
Then go on to list your reasons for wishing to do so, thoroughly explaining each one adding your parents' reflections on your points.
The sort of reasons that will sound good will be that you've found you have no interest or drive for the topics, it's causing you so much stress that you can't focus on your other studies, you feel it has no benefit to your future (if you're using this point it would be ideal to explain a little about your plans, and how history doesn't fit in with them) and that you can't manage your time well if you continue with History.
Finally you need to write a conclusions summarising that the conclusion you've been drawn to is that you can't effectively to continue to study the subject and perform well. Perhaps offer a suggestion as to what you will do with your free time created by dropping the subject, like devoting more time to the arts or sciences.
I hope this has been a help, please feel free to send me a message if you'd like more help planning the letter out.
