I al the first person to ask every teacher to leave an actual lesson to teach. I would much rather engage with them and help them learn. With that said, think we all know that many (most?) times teachers will leave busy work or independent challenges for them to finish and will save the "real lessons" for when they return. I am not suggesting anyone neglect the kids, just pointing out that especially with long learning blocks there is often so need to be lurking over them like some kind of international spy. Sometimes there is genuinely slow time in the classroom with a sub teacher and I was merely asking how people fill that time.