A design brief has nothing to do with design. It doesn’t tell the designer what fonts, colors, or photos to use. Instead it nails down exactly what the design project needs to accomplish in business terms, and as such it provides a translation device for creative people to speak business. A good brief will help you understand the business goals up front, and allows you to explain your design choices in terms that clients will understand. Of course you should briefly (pun intended) explain in your client presentation how your choice of typography, style, and image is appropriate, referencing the brief.
Jacob case’s article on writing a design brief is really a list of questions that should serve as a jumping off point for writing a brief. For class assignments you might try to roll play and come up with a realistic design brief based on what you already to know about the brand.