@strawberrycream sorry, this is old. but its the ghost of nanashima's(the knife guy) twin,(?) sister, nanaka. the one that got abducted and killed.
The teacher that can see ghosts passed along her message to nana that she loved him and is always watching over him. That conversation alerted Kabuto and Ibitsu that the teacher can also see Kabuto. 

 I BELIEVE what happened with the crazy classroom stuff was that Kabuto realized what he was doing to Tsubaki and got super upset, causing everything to go haywire and him saying he needed to get out of here. Since he he had received enough energy to leave Tsubaki's side, he used it to leave and go to the teacher's room. Becuase the teacher is superstitious, he had a salt pile in his room that sucked out Kabuto's strength- causing him to collapse on arrival.

@Murim Lady i aint even gon hold you bro its fμcked up what they be doing to yall...

@Exthralith I dont think lloyd really made her "rotten", the way she confessed it, they both gambled separately. She doesnt blame him at all for the problem, just that he didnt give her enough money to win it big lol. It just seems like 2 people in an unstable relationship bringing eachother down.

@punpun def not, at least, its not likely. theyve been switching urls multiple times since the "death".