So when I first read this, it felt kind of rushed. But then I reread it at a slower pace and I think it fits. Like another comment said, this stuff can just happen out of no where. It doesn't have to be some long thought-out planned thing, and it can really just come out of no where.
It looks like the teacher carries a folding knife around with her, and she's probably become a very resentful and vengeful person. She saw Kaede by chance and just...went for it out of rage.
I don't think Kaede and Tsubasa are exactly "made-up" and all is forgiven now. Tsubasa understands to an extent the terrible things she said, and meeting the teacher and hearing what she said may have helped more. But I wouldn't say she completely understands yet. Obviously Kaede wouldn't want someone to get hurt because of him or to understand him better. He probably thought he'd never see the teacher again.
Anyways that's just my two cents. I feel like it's better digested if you read the chapter slowly.