Maybe you could write something like this letter to attest your mark sheets. Hopefully your principal will be understanding and look into the matter for you.
First Line of address
Name of Principal
Name of school
First line of address
I passed (name of subject) at (level i.e A' Level) in (Month and Year). My roll number is(...) and my enrolment number is (...). Unfortunately, I no longer have my original mark sheet and would be very grateful if you could issue a duplicate.
As I am in need of the sheet in order to further my career, I would appreciate your help in this matter at your earliest convenience.
(Print your name)
I'm sure you can personalise it and make it better suit you needs but that is (I hope) the kind of letter that you could send.
You could try phoning the admin office before sending the letter, they may be able to help you over the phone by taking your details and forwarding the document to you. It would be a quicker process as you wouldn't have to wait for the letter to reach them.