The "Things have been horrible for us since then!" suggest that the bullies have been receiving some kind of punishment since then, which they want revenge for. Yuuka's response, "Well I didn't report you. They came to me themselves, really." suggest that the teachers are the ones punishing the bullies—probably because it's supposed to be taboo to interact with Yuuka.
So far, it seems like the taboo might be for everyone's sake. Yuuka revealed at the
end of chapter 2 that she's responsible for solving the "anomalies" within the school, "to prevent the worst outcome the Troubled Times could have" (which suggests she's the current "mediator"). So the teachers might be fully aware of the supernatural things happening in the school, and the taboo is in place to prevent anyone from bullying Yuuka or interfering with her work, and also to protect other students from getting exposed to the dangers of the supernatural.