Yikes. I've encountered some negligent and lazy teachers as I've grown up, ranging from throwing public resources at my class and leaving us to it, to punishing the class for the mistakes of a single individual (effectively promoting the class to bully the person who messed up), but I don't think I've ever met a teacher willing to publicly embarrass a student over accusations from a letter with no discernable source. Also I get that Japan's citizens tend to be very soft-spoken and avoid confrontation like the plague, but why didn't more students stand up for the student that they clearly knew wouldn't have done anything like what she was accused of?
I feel like I've been feeling more of these cultural disconnects lately in manga. Moments where I, and many people I know, would intervene or do something differently compared to what the characters do. Hopefully what happened this chapter (and will likely spend the next couple of chapters getting resolved) won't ruin this manga or become a reoccurring theme in it's conflicts.