I kept an eye out for "her". This second girl didn't seem to notice that anything was wrong at first. The first girl found other things to complain about.
Later I learned what had happened. Girl #1 had bumped girl #2, and girl #2 responded in a manner not proportional to the offense. Girl #1 was still mad. Luckily, girl #2 stayed away from girl #1.
The first girl was angry about everything. The class was boring. It was even worse because I was there ("Regular teachers are always better than subs. Subs don't know anything."). She said that right in front of me, but she acted like I wasn't even there.
Not ten minutes after this she asked me for favors. After dismissing me as irrelevant? No, I'm going to leave the air conditioner on. No, you can't use the restroom. (Petty? Sure. I get petty when I'm insulted.)
Other than that I did not react to the girl's rudeness (except to mention it in the note). What use would it have served?