Line for instance, can be either traced or freely drawn. Those who DID NOT trace their hand generally scored much higher because tracing is a lower-level skill. As students progress through the year, they gain more confidence in their skills. By the time they take their last benchmark, none are tracing so scores inevitably increase. They learn other skills as well; to shade with color, create shadows that change in tone, to add organic textures, add unique details, and make more careful observations.
My more advanced classes do a similar assessment but they incorporate surrealism into their images. Some regress and trace, but many have moved past that. My adjusting your assessment for the skill level of your students, a similar exam can be given for many different levels making grading easier.
If you are in need of more examples of assessments for art, please visit my "Assessments Tab" to the left for some free examples.