In junior school, we sometimes have people in to give special talks, and when I was a student teacher we had one in my class (year 6) for sex education.
Normally, the class teacher takes a back seat and lets the specialist get on with it, as they are usually experienced.
So that's what I did, but the class rapidly became rowdy, and I assumed the guest would know how to handle them, but she didn't.
Being just a student on teaching practice, I wasn't sure what to do, but I stepped in and quietened them down.
She then continued, but they soon became rowdy again, and said she couldn't carry on and left.
I then told the class off about their behaviour, calmed them down, and got them working in complete silence.
A few minutes later, the Head stormed in, and was obviously surprised by the silence and everyone working.
She then gave them a good telling off as well.
I had been given no guidance in advance about how to handle the guest visit. I assumed that, as she gave the same talk at various schools, she would know how to handle a class of 30 11-year olds.
Apparently this was not the case and I should have controlled discipline from the outset, but no-one had told me this, and as I said, the teacher usually takes a back seat and doesn't interfere.