I agree with Bwoodbury. A meeting should be called to discuss the concerns of the Director and TD. If I were in this student SM position, I would prefer the meeting be with the TD, Director and myself, not with anyone else. Having the ASM present could only make the situation more awkward for me, and possibly the ASM. After the initial meeting alone, the ASM could be brought in to help discuss solutions. It must already be strange for the SM, since this situation seems to be being discussed with everyone but her.
It is a college production, and students can never learn to call shows properly without having the opportunity. If this is similar to any other college situation, the SM and ASM were chosen for their respective roles by the TD, which assumes that the TD felt those people could handle the SM and ASM roles they were given. And should they decide to swap the positions, it may feel to the original SM that she might as well just leave the theater department. Students have to be given a chance, and if guidance is necessary, then the TD must step up to the plate and assist. Or, if you are the one teaching the SM, then you may need to step in and help get her on the right track.
Personally, I never took SM classes and only learned how to SM by observing others. My sophomore year of college I was given a series of One Acts to SM, and while I was scared, the TD was present during the paper tech (since the LD & SnD were also students) and helped me fully understand what I was doing. The faculty must help take responsibility for these students and help them gain the knowledge they require to do their jobs - instead of tossing them to the side because they feel they are "not on top of things."
But that is only my opinion. And if the ASM and SM are your responsibility (you say you're helping teach them), then you may need to step in and find out exactly what's wrong. Hearsay helped no one. Perhaps you should go into a meeting with the TD and Director to get to the heart of the matter first - and see what can be corrected or assisted before things get ugly and people's feelings get hurt.