Make the original assignment clear and specific to students. Instructing them to draw an arranged still life may produce finished images to assess, but it's difficult to see how well the class understands specific concepts. Telling students to depict that still life using lines of varying strengths, masses of shadow or as a reversed image on black paper will give you a clear indication of how well the class understands the underlying concepts that go into creating a finished artwork.
Observe the process, not just the product. A student who has struggled to understand foreshortening may reach a breakthrough with a finished drawing that lacks overall proportion, but showcases a new understanding of perspective. Conversely, a proficient student may turn in polished, but formulaic work that doesn't require much problem-solving effort.
Compare students' development with their past work to see their growth. Each student starts at a different level of artistic expertise, so assessing their work as an outgrowth of previous works instead of comparing all student art to a single ideal gives a truer indication of their progress.
Discuss assessments of student art in neutral terms. Students put a great deal of themselves into their work, and a critique that includes negative terminology can sting on a personal level. Frame the assessment in terms of how successfully the student met the lesson's objectives instead of in terms of right and wrong answers.
Involve all students with a critique session. Students who didn't contribute work cannot participate in the group critique.
Display all art in a similar format and with good lighting. Every student's work deserves scrutiny under proper conditions.
Couch the assessment in neutral terminology, pointing out striking features or overall impressions rather than passing judgment on whether a piece is good or bad.
Let each speaker offer a critique unchallenged to ensure that everyone in the class is heard. Although dialogue has its place after the critique, make the assessment itself a series of monologues with the artist remaining silent to absorb what others say.