I think... she is a ghost, or some kind of spirit from her friend that really died a while ago, that would explain why shiori was impressed when watching the student register
Hoping she doesn't end up actually getting eaten by the black haired girl later on, wouldn't surprise me but also wouldn't be a very satisfying way to conclude it
I have a gut feeling what may have happened but I am so looking forward to the next chapter. Thanks so much for all the hard work in getting this to us