Musically Deranged

We have a music teacher at our school with a great love for music, but what has been demonstrated as an incompetence with teaching. I happen to have music on Fridays. Well, last week, though the teacher mentioned nothing to me, my students told me that one of my students was playing "guns" with the rhythm sticks. Today, I decided I would stop by about ten minutes into the lesson to see exactly what kind of mayhem my students were causing. First of all, I immediately noticed the sudden change in the atmosphere when I opened the door. If you did not know this, the sight of a teacher (especially their own teacher) stops third graders in their tracks. Several students were looking at me from their seat, others were standing, and ONE was on the floor!

"Is it time already?" asked our obviously senile music teacher.
"No, I just decided to check on them." I stated in my best "I-am-very-upset-right-now" teacher voice.

Since it didn't seem like the teacher was actually in the middle of a lesson, I spoke to several students about sitting correctly in their chair, not touching their neighbors- basically correcting the majority of the misbehavior I was noticing. I pulled Arnulfo, the student who was on the floor when I opened the door, and spoke with him outside.

"What did I tell you before I let you in this classroom?"
"To behave."
"What are you doing now?"
"Not behaving."
"Do you think you'd be able to control yourself if I put you back in there?"
"Yes."
"Are you sure? - because I have no problem with having you set this one out."
"I'm sure."
"If I hear one more thing about your lack of self control, you will NOT want to know me."
"Okay, Miss Sells."
"Do you know what I mean by that?"
"Yes. You'll, well... you'll basically attack me."
(I had to suppress a smile) "That's right. Go back in there."

What do I do with a class of students who will behave with me and the other specials teachers, but not with one? And on a Friday afternoon?! Aaaah!!!