Same class... different class?

I had an unusual situation yesterday. An S1 class I regularly take was foisted upon me as a cover class due to a school-wide project with S2.

Now, they're a good class. I enjoy taking them, though they're probably the most "boisterous" of my classes at that level. They still produce good work and we get on well.

As such I was staggered with their behaviour in my cover lesson. They were supposed to be in PE, and were instead transferred to me in a Computing classroom to do written work. It was chaos. Chatter, misbehaviour, failure to follow instructions, messing around, paper all over the place, "sir, he's stolen my sheet"... You name it. Absolutely unreal.

I had four of them out of the class to talk to, including one I'd never had to so much as had to point at before. One ended up going home with a punishment exercise for continuing to mess around immediately after I told him to stop and was still looking at him... then laughed when I took him outside for a lecture. Again, a child I'd usually have little issue with.

By quirk of timetable I had them in my room the next period for Computing. The lesson went very well indeed, although I did have a right go at them before we got on with the task at hand.

I don't know if this is common or not. Same kids, same environment, same teacher... different work to be done - and it all goes mental.