Huh? I think what you're saying is correct, but this sounds very strange.
How long was Eiji being bullied for then?
Wrong. This is not even true in real life. There are degrees to wrongdoing that address both intent and action. Ex: A case of attempted murder does not necessitate an equal penalty to murder of the first, or even the second, degree.
If Eiji died, this story and its consequences would be a lot different from it is now. But he didn't. So how does "any" punishment fit? Miyuki didn't intend for her to kill himself, nor did she direct him to do so. She is an accessory to his attempted suicide. This, apparently, goes both ways, as Miyuki attempts suicide herself? So if she dies, this means everyone involved in it deserves "any" punishment too, right? Including the teachers who failed to do anything about it? Know that this is also bullying and that "she started it" would not absolve the crime. An eye for an eye leaves the whole world blind.
Had Eiji died as a result of her actions, she would be punished according to that reality. But that didn't happen, so punish her in proportion to that. Lastly, I want to say that the discussions on "mistakes" and "wrong choice" are thoroughly lacking in nuance. A mistake isn't a matter of conscious decisions. A person's awareness contributes significantly. Miyuki is a child. Regardless of what they might know, they are not expected to have the emotional or mental competence of an adult. It is for this very reason that we don't take a teenager's word as consent on its own, otherwise sexual predators could get away with just that. That's just not how the world works.
I'll say it again: Miyuki is a child. Clearly a stupid and easily manipulated one. But that doesn't make her a moustache-twirling villain or a psychopath who delights in the suffering of others. Whatever you want to pin on her: cheating, accessory to bullying, a proper man or woman would shake their head in disappointment at her bullshit and tell her to wise up. It is also within their rights to wash their hands of her situation, as they have no further obligation. Teachers should reprimand her, then give options and support within their scope. Her life is barely starting, and one mistake shouldn't be the end of it. She has countless options ahead, good or bad. Someone who would celebrate a child being driven to suicide for something like this in real life would be sick in the head. Good thing that this is revenge fantasy porn.