Still, follow the lesson of the teacher...
It takes real effort to get such an environment in your dojo, but when you get it, it works really well. Off course this is a continuous process.
I think it is the responsibility of the teacher to make this very clear to all students. It forces students to think about what the teacher is trying to achieve so they do not distract eachother with other issues. I do correct students that correct others on aspects outside my lesson.
I have encountered people on the tatami that simply did what they wanted to work on regardless of what the teacher is doing. They undermine the entire lesson of the teacher and even worse they often start correcting others on things they work on....very bad.
That's the dynamics of the whole dojo. How it will work out depends to a fairly large degree on how the teacher handles the issues you mention.
Very new beginners though (if the class is mixed so they're not in the beginners' class) might need some help with other issues than those the teacher mentioned, since having somewhat right position etc is a prerequisite for working on anything else in the technique.