Ah writing sick letters is easy. I used to write fake ones to get out of school all the time :) The trick is never get your mum to sign anything, otherwise they will have a signature to
How to write a sick note
To [Principle's Name],
I regret to inform you that I have been taken poorly, and will be unable to attend my course from [date you will be missing out on].
Having consulted with my doctor, it turns out I am suffering from [insert exotic disease name], and have been recommended a period of no less than [how long you will be of] bed rest.
I intend to work hard to make up on anything that I miss, and am eager to return to my course once I am in better health.
If you're going to hand it in in person, make sure you look like you're going to die. Even if you are genuinely ill, when handing in the letter you will be scrutinized to make sure it is all kosher and true.
If you go overboard, this leaves no room for doubt that you are faking an illness or whatever.