Outline your planned plot for your script before you begin writing. Knowing what points you want to reach in your school play can help you keep your plot on track while you write. It will also give you an idea of any research you may need to do.
Research the appropriate format for your script. Scriptwriting has certain conventions for noting dialog, action and setting, and following these conventions can make it easier for you to edit your play later on.
Write your play. You will have to commit time to finishing your script. While you may be able to write quickly, a full-length play can be over 90 pages. You will typically need to write a page to a page and a half for every minute your actors will be on stage.
Edit your play thoroughly. Reading a play out loud to yourself will help you catch typos and word choice issues. Editing your play before asking readers to look at it will improve the quality of critiques because they can focus on the plot and characters, rather than distracting typos.
Ask other people to read your script, both individually and out loud as a group. If you can get a few students of the appropriate age for your school play to read it, you can develop an idea of any big stumbling blocks they may face. Rewrite once again if your readings indicate flaws in your play.