You can easily see the students' % mastery through unit and course and mastery level for each lesson, as long as mastery is available (courses in beta stage don't have mastery enabled yet).

To see % mastery over the course for each student, go to the teacher dashboard and click on the class.  Click progress, and at the top of the page will be a graphic for class mastery of the course.  Click show students on the right, and you will see each student's individual mastery % for the course.  To see the students' mastery levels for a particular unit, scroll down and click that unit.  When it opens, the students' individual unit mastery % will display above the graphic.

To see mastery level for each skill, click Activity Overview, then Skills.  Click whichever unit you want to check, then a particular lesson.  It will show where every single student is, and you can assign the lesson again to any student who is struggling or needs extra practice.  If you right click view exercise and open it in a new tab, it will show up with a sequence on the left that will usually include videos and possibly articles; clicking on those will allow you to assign them using the bar that appears at the top of the page.

The skills page is a better indicator of student level than EP earned in the course.  It will show you which students attempted a lesson but couldn't answer at least 75% of the questions correctly; EP won't reflect that data.

If you need further help, please let me know!  

I chased up P. Rossell's suggestion about going through Activity Overview, then Skills. f you expand on an exercise you can get a list of which students are on each level, including mastery. This exactly what I was looking for, although it's a bit more cumbersome than the old deleted facility. However, it does provide for assigning each exercise to individual students who have not achieved mastery as appropriate.
I cannot thank P. Rossell enough for pointing this out; it will do the job for me in my individual programming for my highly differentiated class.
Thank you, thank you.
Colin Appleton, Bangkok

Hi Colin,

I'm so glad that it shows what you need!

As I said, it's not a perfect solution.  There are a ton of courses (all of the ELA grade level courses!) that are still in a beta stage, so that view isn't available yet.  But it works for everything that has mastery enabled.

I'm glad to help, Colin.  I wish there was an easier way to do it, but for now, this is what we have.

all interested - please upvote this topic in Feature Request with similar request