It must be true what they say about psychopaths: They might outwardly appear like the everyday jovial neighbours. I wasn't expecting the teacher to harbour vengeance, but on the other hand, his attitude always seemed far more given up than Yuri's. So, it makes sense he would rather do something more commonplace, like avenge Momoka, than seek mystical diaries to bring her back to life. His speech to Yuri must have been to either keep her out of trouble or to keep her out of his way. Though it's not like we'd have already seen what exactly he is planning, and having Ringo present makes it even more strange considering she's Momoka's sister. It would be odd if he purposefully hurt her while doing something for Momoka (in his own opinion).
The penguins acquired a stable companion in the form of an octopus?