Very often, interviewers will get a good sense of the art teacher's effectiveness by looking at samples of his or her students' work. The samples should not only include your students' best work, but they should also demonstrate the breadth of your teaching ability. In other words, if during your student teaching you taught students painting, ceramics, weaving, photography, and found-object sculpture, your portfolio should, ideally, include samples covering all of these mediums.
Because students almost always want to take their art work home, the preservice art teacher should plan to take photographs of his or her students' work, rather than keeping originals.