Honestly I don't get most of the school things, other than "Sabi has trouble in school"
That one, I'd say it represents Sabi being alone and marginalized in school?
Also that she's a monster, an experiment like the others, that has been dropped there (that's what I get from those jail looking shoelockers)
I think it's an interesting detail that the room in >>2703
hosts one of the calm twintail Kaibutsu before the Rust chapter. But maybe it doesn't mean anything. Though, I recall there being quite a few empty other rooms in the third floor that lol could've chosen to put this Sabitsuki in, so maybe it does mean something…