Question

Which aesthetic theory do art teachers at school use to grade their students

Answer 1

It often depends on the type of art that the teacher was looking for. For example, if a ceramics teacher was looking for a coil pot, often times they will just hand out a rubric. Typically the requirements on art rubrics are loose- otherwise everybody's work would end up looking identical. For example, one requirement could just be "a couple rows of different coil designs" for a coil pot for full points on that assignment. Art teachers also grade based on a self-reflection form students may fill out. For more abstract pieces, the teacher might just grade based on why the student designed their artwork like that.
Hope that helped you.