Do you work during a prep period?

Question for long time subs or those new to the game; do you work during prep period if the teacher writes it in the lesson plans?

I subbed for a 4th grade class for a new teacher and at a school I've never been to. There was prep period right before recess when students went to music. It was a nice 55 minute break. I usually bring a book with me for situations like this so I sat down to read. But when I glanced at the lesson plans, the teacher written "organize turned in students work" during the prep period time slot. I was confused because the kids hadn't turned in any work prior to music class. I looked around the room and there were several stacks of papers in trays, on countertops, near the printer, etc. that seemed to be organized so figured maybe the teacher had copied and pasted an old sub plan which didn't apply to me so went about the day.

This is the second time I've subbed in a class where the teacher wrote down something for me to do during a prep period. The other time I was told to grade a multiplication math quiz students had just taken. I graded a couple before giving up. It was one of those timed multiplication sheets students do in a minute or less. (those anxiety/stress causing quizzes should really be banned, imo) Anyway, some of the kids writing was so sloppy I couldn't make out what some of the #'s were. I did leave her a note explaining why only a few were graded.

Anyway, I'm curious to know have you ever been given a To Do List during a prep period and if you actually do it?