A) a failed attempt to hide the stain
B) Her getting in his face because he got one of her daddy's good shirts dirty.
[I'd like to take this opportunity to restate my objection to the oncoming torrent of liquid manure. As eeknight said, nobody in the real world would allow five-or-six-year-old children access to paint that wouldn't wash out or to turpentine. Since Lynn didn't do the research, we're in for the denouement of an idiot plot wherein Mira is cast as Evil Incarnate for disciplining Deanna.]
Panel 1: As she mops up the mess Mike made, the art teacher reassures him that he shouldn't worry because something gets spilled every time they have Art class. That's why they wear painting shirts.
Panel 2: Deanna butts in to whine about how that was one of her daddy's shirts and that she loaned it to him. Teacher responds that it's an old shirt and the stain will wash out anyway.
Panel 3: Miffed that the dumb Patterson kid soiled such a holy relic, Daddy's Little Pisstank huffily walks away making a self-righteous TSK.
Panel 4: This causes the Poster Child for Masochism to thoughtbubble that a prickly annoyance TSKed him.
Summary: This is Lynn's way of explaining the real source of the conflict between them. Deanna-fat-as-a-pianna was his way of defending himself against a twit who wanted to set him up to get punished because her mother was evil.