How Do You Write An Application For Joining A School?

You do not mention what kind of school it is, a technical school, a college, a university, a business school or high school, who will all have their own norms and proceedures.
However, generally you could start by having your name, address, and the date, in the top right hand corner of the page.
Then the address of the school, and the title of the head of the school your are wrting to, on the left hand side, under that. For example, The Rector, The Headmaster or The Principal, depending on your situation.
Then the subject, for example: "Application for entry to Greenfields School.
Then the body of the letter, stating your history up to today and your last set of results.
You can end off, by stating briefly, that you feel you will contribute positively to the school, and that you look forward to hearing from them.
Then sign it , Yours Faithfully, with your name under this, and then put your signature, with a pen, beneath this.

Don't... Most schools these days don't mind if you just turn up one day and sit in class, perhaps make Friend's with a truant and use his/her information. Good for you and even better for them