“When you begin a new iMovie project, you have a choice of creating a movie or a trailer. A trailer follows a template to create a Hollywood-style movie advertisement. iMovie for iOS has 14 trailer templates, and each has its own musical score and graphics. To make your own trailer, choose a template, insert your text, and fill each shot with a video or photo.”
With any video project, it is advisable to have students to develop some sort of storyboard to help guide the thought process along with the evolution of the final product. Here is a great resource with PDFs to help in the planning process for iMovie trailers. Also, some great advice on approaches to learning.