@Psychronia
Basically, Japanese people are really responsible people, or at least, Japanese people expect someone to take responsibility when something bad happens. There is no 'blameless incident'. When something bad happens, someone has to take the blame. Yes, if there is a natural disaster, someone has to take the blame too for not preparing for it.
If she didn't say anything, the school can sweep this under the rug and silently admonish the girls who did it. Sometimes a suspension, sometimes they would get kicked out of school. But since she did say it in public, public outcry will demand a sacrifice, regardless of the situation. This means that one of the teachers responsible for the trip will have to resign, regardless of the outcome. Unless there's a really strong reason why Jimi must stay in that school, Jimi will most likely be the one who is forced to resign. That's not the worst of the outcome, but talking about the cascading effect will take too long.
Now let's talk about her. By doing this, she has smeared mud on her own name, at least where schools are concerned. If she wants to change school for whatever reason, schools will most likely reject her application for transfer. They would think, "She did it once, what's stopping her from ruining our reputation next?" Technically public schools in Japan can't discriminate based on past deeds. However, schools can refuse her application by giving bullshit excuses, such as her failing the entrance exam or the school is full or the school doesn't allow part-time jobs or the school doesn't have the facility to accomodate teen idols or any other excuse and that refusal will be valid.
Had this just been her saying stuff in the newspaper or blog, she would normally be suspended. But since this has become too big, the school can't do that, since that would be similar to admitting that they're trying to silence the incident. That doesn't include how people would blame her for 'shaming her friend'. The Japanese people really value privacy.