This is by far so frustrating.
My first approach would be to go to an advisor. Being academic, I assume there is an advisor/supervisor and this is being graded to a certain extent. My approach would be, "This is the situation with the other students SMs/Director, etc...how do you suggest I approach this?" similar to what you are doing here. I've been in this situation, with a designer in an academic setting. A designer who was a friend of mine, and rather go the "friend route," I chose to address it in a more distant but professional way, as best I could. (What differs here is that the designer and I were both non-students, hired in professionally. So, even these sort of situations happen to those who should know better).
How approachable are these other managers? Can you address them? Is this affecting the cast? If remarks are made about the need for "you to be on book," etc., there might need to address that their lack of focus should both not be blamed on you, as we all have jobs to do and need to focus on rehearsals, etc.
This is so frustrating. I know, it's easier said than done to approach this. Unfortunately, not all stage managers practice the same way, and some appear more responsible than others.
I would say your best bet is to address it to them if not to go to an advisor for assistance, as this is a tricky situation....
Let us know how it turns out, please.