I encourage my students to save all their artwork for a considerable length of time - at least until they have a body of work that demonstrates a measurable level of achievement. Some drawings, especially experimental warm up exercises are good to keep as reference for future works. I find my warm up drawings on newsprint hold interesting textures, treatments, and elements that can spark an idea for a serious piece of work. The images can be expanded to create a complex finished piece, or combined with other work.
Though a student most likely won’t appreciate his or her beginning work, it is vital to retain it to show the artist’s growth. Work which an artist would consider unsatisfactory usually has some aspect of it that is done with finesse, or contains a treatment, texture, or delightful “accident” that will inspire another future work. Even the Masters of drawing didn’t always like the work they do. They sometimes have been known to crop a drawing or painting to leave only the portion they wanted to save.