it is just a guess.. but i think there is no choice...if i follow..principal´s lines correctly.. as "remains as a teacher"..
i think there is a chance..of she diying here.. ergo,,, koro broking the promise of not harming a student...
he will be forced..to a death battle.. you just cant scape from her, goofing around .NOT THIS TIME..
jokes are over , this is more real than itona and the god of death ..
could this be.. a match 20 battle to death? ...also i got the feeling she organize everething on that little room
as a trap to kill him ..EVERY GOD DAAM LITTLE THING..from the beads..to the smell of fish...so he cant "smell traps"
she is the perfect killer indeed,,,planning all this..long time ago..each day on class,
with nothing more in mind...than kill koro sensei, and at the same time remaining as a background character,
honestly i didnt pick up all the hints..til this time..but yes it was all there.