AHAU: Art Professor Cas
Details: The fic starts with Castiel setting up for his new art class. The students walk in and one of them is Dean. Dean goes up to Cas to get his art supplies because he wasn't allowed to get them on his own time. (Dean is on parole, therefore he had restriction placed on him that made it difficult to get the supplies he needed for the class.) Soon the class starts and Cas asks to students to draw a still-life portrait of something in the room so that he can learn their skill level. Cas walks around the room, giving critique as needed when he gets to Dean. The first thing Cas notices is that Dean drew an object that most of the students had ignored. (It was a ornate handle of a jug/mug, I think.) The second thing Cas noticed was that Dean was very talented, with only minor errors in his drawing.
Later on, Cas gives the students a home assignment and at some point suggested to Dean that maybe he should draw a car part for the assignment. (I forget why Cas made that suggestion.)
The only other detail I remember is that Anna taught an advanced art class and Dean might have been recommended into taking that class next semester.