Drawings are the core of a strong portfolio
You might have 15 digital paintings with an impressive amount of technical skill, but none of that will matter if you have poor drawings. In terms of drawing media, the vast majority of high school students are creating tight, conservative, photo realistic pencil drawings drawn from photographs.
Drawing is not about just copying a photograph as accurately as possible; we now have cameras that can do this instantly with incredibly precision and quality. Ask yourself, what you can express with your drawing that a camera would not be capable of producing by itself. What types of marks and imagery can you create that are unique to your drawing materials?