Remember this event that happened with this class? Well, a couple parents did call me later and we all had productive chats. One parent showed up to the last art class of the session and made sure things went well.
And they did. All the students were excellently behaved. All had fun. All worked hard on a lesson that I had waited, and waited, and waited to teach them because I wasn't sure if the class, as a whole, was mature enough to handle it. All were in awe of their art portfolio, which was handed back to them on that day.
Thus, I'll be back at this school in a couple weeks when the new session starts up, with carefully trained students who have finally gotten used to me and my tendency to not let them get away with anything less than their best.
Goal for next time: cultivating self-motivation in students to not allow themselves to get away with anything less than their best.