Whether you get to wear the hijab or not, being very respectful of the rules in your school will help you in many ways and it will help others be more respectful of the reason you want to wear it.
So, in your letter you should express that you will certainly abide by the rules but that you are asking for help to understand why the rules do not allow what you want. You can then make a respectful request for a change in the rules.
Remember, though, you must truly be respectful and not just use the idea of being respectful to manipulate a change so you can have what you want. If you are just being manipulative you will harm first yourself, then others in the end.
If you are deceitful about your attitude on the matter of following the rules in any way you will greatly degrade yourself so give the whole idea up before falling into that trap.