给英国学校校长写信（非诚勿扰）

Hello,
In terms of formality of the letter, the content has to be relatively formal, including the header(Mr Headmaster) and the footer(Regards/Your sincerely), and then your name at last. The comma have to been used instead of the colon sign. But if you are the student who are willing to attend the school, then normal writing style will do just fine, although you have to clarify your identity. They won't mind as long as it makes sense.
I am a English A-level student as well, and I wrote to my headmaster to explain something serious before, so I have this kind of experience, just write what you want and put header and footer down, you will be fine. Good luck.