One thing to remember is that we all have different work styles. Has this person ASM'ed before? Is she used to SM'ing her own shows and hasn't ASM'ed (therefore being unfamiliar with the "assistant" or backstage part of the job)? Although it is a nice thought, not everyone works well with everyone else. Even experienced, professional SM's/ASM's sometimes have such different styles that it is almost impossible for them to work together. So if she has any experience, consider that this might be part of the problem.
If she is unexperienced, it may be that she just doesn't understand what her job is, and how things need to be done. We all understand that you shouldn't text message during rehearsal, but someone who is new to the process maybe doesn't. What might be the best thing to do, especially since it is an educational setting, is to sit down with her (and with an advisor/faculty person if you feel it is necessary) and very clearly define what your expectations are from her in how she should behave in rehearsals, how you would like jobs to be done, and what the general role of a ASM is. It is very possible that no one has ever told her these things clearly, AND/OR that she realizes that she is completely over her head and doesn't want to look bad or admit that she doesn't know what she is doing. We all had to start somewhere, and as you said, not everyone is cut out for stage management. If your production has the time and the ability to replace her, give her the opportunity to make the choice to commit to the production or to say "I tried, I'm sorry, but it's not for me" and to walk away. That way everyone has a clear idea of where everyone else stands, she will know what is expected of her, and you don't have to worry about your actors being squashed by a 400 lbs wagon. Then hold her to those standards that you set by a certain date, and if she doesn't hold up her end of it, you will have been clear about why you have to replace her.
This is not an easy situation for anyone, and I hope it works out for you!!
SSM