Considerations regarding the publication's intended audience is perhaps the most important thing to keep in mind when writing an editorial. This can help shape the editorial itself, as well as determine the mood and style. If the editorial is for an official school newsletter, the editorial should remain professional and shouldn't work to shed negative light on the school itself. If, however, the editorial is for a student newspaper, something more comical or inflammatory might be tolerated, such as an article talking about student sexuality or a piece that makes fun of a certain school tradition. Reading past works of a particular publication can give you an idea about what types of articles and moods are most appropriate.
Editorial writers should also consider the school itself: its location, the age of students, and the issues it currently faces. A publication coming from a public elementary school in rural Alabama is probably dealing with far different issues than a private college in a large city. Simply considering the school itself can inform the kind of editorial that you write. If there are any topics directly relevant to that school, these are probably more appropriate stories for editorials than, say, a nationwide issue that doesn't have any specific relevance to the school.
Some editorial stories may be more appropriate for different publications. For a bimonthly school newspaper, for example, it might be very appropriate to write a lengthy article regarding an opinion on a long-lasting topic, about such things as falling graduation rates or the effectiveness of current teaching methods. This kind of editorial would have to be very different than one featured on something like a school publication website that is updated weekly. For this kind of editorial, an immediate issue might be a more appropriate topic, such as an opinion piece on the need for more parking spaces or too much littering on campus. The length, scope, and long-term relevance should be informed by the medium of publication.
Finally, if you're going to take the time to write an editorial for a school publication, you should try to pick a topic that you are truly passionate about. Chances are, if you feel strongly about a certain issue, others feel strongly about it, too. Writing an editorial about such a topic helps to open a dialogue and might even sway opposing parties to consider the issue in a new way.