Oof.
Is there an established protocol for being a PA at this organization while enrolled as a student? What I'm asking is this: is this PA job viewed as part of your coursework? Is the LORT attached to your university? And - is this the sort of information the SM/PSM could get without your being directly involved? If it is, then my vote would be for you to avoid spilling the beans. If the SM is going to find out from another senior member of the company and/or school, that is a more appropriate channel, in my opinion. If you make it your project to tell the SM what your fellow PA considers (rightly or wrongly, not for me to say) a schedule conflict, that may reflect a bit un-fabulously on you. (I am assuming that you and the other PA are both relatively new to working with this SM - apologies if that is wrong...)
That said, this does tell you a lot about what the other PA can and will do. While it certainly isn't the be-all and end-all of her commitment to the job, I would take it as a lesson outside of the classroom - she opted to go for the dance class, and you opted to tape out and otherwise prep the space. Use this, as objectively as possible, to look at how your particular style of stage managing has already developed.
And - this may give you a golden opportunity to build a working relationship with the SM on this show. I've found that, if you make yourself available, it will pay off.
Best of luck,
Meg