So, not to harp on the whole "The teacher isn't actually subversive and classical nudes are, well, classical." thing too much, but there's an interesting parallels between Gin's stated desires, at the start of this chapter, and the solution she presents him at the end.
Gin says "... still life art is monotonous because of how static it is." and later "What I want to draw is something that's not... Not this dull white. It's more like... Transparent. Something that does not move while also in the middle of motion. It's not exactly something you can draw with black pencil lines either."
To me, the obvious answer to that desire is Dreams, ideals, some sort of message or concept that's more than just a recreation of something in the physical world, and graffiti is almost the perfect medium for that type of thing. It's not black and white, at a minimum taking on the color and characteristic of whatever you're painting on; it can be heavily stylized and convey a sense of motion, without ever involving a subject that actually moves; and, oh yeah, it's not done with just a black pencil on canvas.
However, by the end of the chapter, he's taken by the art of Paul Delvaux, who's subjects are described as women who "gaze languidly into space, as if hypnotized." In other words, women who are depicted as lifeless, without motion or dynamism, effectively transmuting them into static still lives, that might as well have been plaster sculptures.
Now, that particular line comes from the T/N notes, so it might not be what the author was going for or as strongly present in the story. At the same time, those parallels are striking and this is also a chapter that showed the teacher getting rejected from an art competition and saying she always wanted to be an artist, not a teacher, perhaps hinting at serious flaws in her approach to art. It's probably not the case, but I'd love it if this set up an arc where Gin realizes that what his graffiti was actually missing was an intent or message and that the teacher's art has a similar flaw, despite being a different medium.