Assessing creativity is akin to watching someone who has all the parts to make a car, and then grading them on how they go about putting it together. I usually grade the parts, then help facilitate the reassembling. I think creativity grows with knowledge. Oh, you can see a kid who is very creative, but that doesn't necessarily mean they "know" what they are doing, or even can speak about the choices they are making. Once we can get students to articulate their thesis and their choices then I think you can grade or assess "creativity". That said they need to know that there are choices, and that choices in art, just like choices in life, have consequences. That is what I did as an art educator, facilitate their processessing of information to make the choices necessary to fulfill their thesis. In this conversation when I say choices, I mean the elements and principles for a start.