You'll need to write this type of letter with loads of facts -- state what you're charged with and state why you are not guilty. Be sure to provide proof. For example, if it's a matter of not being where the offense took place, be sure to state that and give names of people who can prove you were elsewhere.
Here are some websites to help you learn how to phrase such a letter:
If you want one of us to critique your letter, do not post it. Please send it via the form at the following link:
You really ought to examine the evidence offered against your case first. Then, the letter can address and perhaps refute that evidence.
If it is a matter of character, which it often is, you will need character reference letters from classmates and instructors.
Examine the case against you in detail, and think like the ethics board: what would they decide?
My experience with code of conduct appeals is that the appellate often tries to defend actions by claiming good intention, or accidental violations. Dont fall into that mode, it is ridiculous. We all make choices, some good, and some bad. Never confuse the two. Your judges won't.