Jot down a few reasons why you like your school. You should have five to seven reasons when you finish. An op-ed, or opinion piece, needs to be factual. This means that you should focus on concrete reasons why you like your school rather than give anecdotes about your school or emotional statements without factual backing, such as "I like my school because it's fun."
Write an outline. The first part of each section should be a reason why you like your school, followed by one or two points explaining or exemplifying the main point.
Write a rough draft. Remember that you're writing for a newspaper -- the sentences should be short and declarative, in the active voice, and with only three or four per paragraph. So, each paragraph will ideally open with a sentence that gives a reason why you like your school, a sentence explaining this reason further and a sentence with an example to support it.
Edit your rough draft for style, grammar and general readability. You need to make sure that someone unfamiliar with you and your school will be able to read it.
Wait until there is a high-profile education-related story in the news. Newspapers are more inclined to publish op-eds on subjects that are current, so your essay on why you like your school will fit in much better with a story on education than, for example, a story on pig hunting or crises in the Middle East.