Oh nice, it's back. Now for that 9 chapter crunch to get the whole thing out, right?
i think it's just way too obvious that it's the himeno saeki girl right? that or the culprit was the brother all along because he went to live alone. the reason sounds redundant but I feel it's a twist that could be done for shock value
The setup did feel like it's supposed to point at the brother being the cause from the first few chapters, what with him thinking that to start with, the angel placing him as a teacher to fit the "the killer is in this class" line, and all that. Though that doesn't explain the blackmail texts, so maybe it's something else after all.
Maybe it's some kind of double twist where the sister is the culprit, because that's how suicide works? Hell, maybe it really is her behind it all, hoping that escalating bullying would cause her brother to move back in with her, or make him bring her to live with him.