when did koro send pai away

Are you referring to the movie or book version?

koro doesn't send pai away in the movie, Nanny Flowers does and that is because Koro was having a hard time after he realized that all those boys he took time for to train weren't going to be good enough to be chief and Nanny thinks it wouldn't help if he had to be with pai all the time. (Because they were mad at each other) so she send her to her uncle (Rawiri) for a while.