After I started teaching, I began to understand why my own teachers had always conducted classes in a certain way. I always thought of myself as a good student - I did my homework, and I was never disruptive. Nevertheless, I did so many little things, that broke the flow of the class, without ever realizing it.
One of those things is covering your paper as you work on individual assignments during class. When I wasn't sure of my answers I always tried to hide my paper as the teacher walked around the class. I felt embarrassed if my answers were wrong. Now I realize how important it is for the teacher to see how the students are understanding the material. As a teacher. if I see that the students are struggling, I think that I have described the material badly. If I see that several students are struggling, I might interrupt the activity and explain the material again. I only one or two students are struggling then I try to work with them, so that by the time the others are finished, everybody is on the same page. Either way, the reason teachers assign individual work in class is to have the time to make sure every student is understanding the lesson.
On the other spectrum from not understanding - is working ahead. When you understand the new topic and catch on to the concepts, you think - I'll just keep working, the teacher is just repeating herself anyways. But for a teacher, one of the most important factors is to keep everybody together. When students work ahead, they stop paying attention, and they might miss directions on a group activity or something like that, causing unneeded breaks in the class, and taking away attention from others.
Pay attention to your teachers! They are there to help and only help!