Hmm. I haven't actually done that before, but for a high school age review, I would consider:
the concept: is it clear what the artist is trying to do?
the execution: does the piece look carefully executed?
colour and perspective, theory, etc: a grasp of the fundamentals of the materials and how to mix or apply colour?
Depending on the time you want to invest, you might want to find out what the assignment criteria was, so your comments can relate to those.
Let us know how it goes!
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