Okay, so I'm going to go through the entire creation of the evil Heavenly Host Elementary, through every detail. Enjoi.
SPOILER ALERT SPOILERS SPOILERS EVERYWHERE
Magic is powered by emotion. When someone has the will and want to protect or help someone, it's not nerve impulse that helps that person, it's the body's wish to do so. The same thing can be said with magic, for if you wish to use magic to protect someone, and you have strong feelings towards doing so, the outcome will be your emotion.
But the same is done through the opposite of emotions. Hate and Anger can lead someone to kill another, Greif and Agony can lead one to kill themself, Sadness and Loss can lead someone to look for an escape. When one has strong negative emotions towards someone else, then negative magic is directed towards that person.
Most Protagonists use the will to protect someone, the innocent people of the world, and use those emotions to power their fight against evils. While at the same time, the Antagonist of that story would do the opposite with negative emotions, thus causing clash.
In a twist, emotions can easily lead a person to insanity. Magic can do the same, if used wrong. Insanity is the strongest emotion, thus the strongest magic.
When Sachiko (And Yoshie) was murdered by the principal, she built up hatred for him over a course of several years. Do to improper burial and mistreatment, her soul was forced to remain inside the school. Each day her hatred for Takamine grew ever more, and it seeped into his very being, corrupting his mind. That blood was on his hands alone. Soon, Sachiko's hatred and sadness welled up enough to give her the powerful magic of hate. With her physical form, she could do whatever she wished. And she wished to ruin the principals' life.
At the same time, the "White Sachiko" inside of her still cared for her mother. She wanted to make her happy in some way, as she, too, was trapped inside this school because of the principal. Those two conflicting emotions gave birth to evil, and she used her form to kill those around her, for the sake of her mother and for the sake of ruining the principal.
Soon, her Emotions took over. This is the point where Magic consumes her very being, and she becomes insane. She can only remember what she has done in her ascension into such power, which is kill, kill, kill. After her job with the principal was done, she forgot that she ever wanted that, and continued to do the only thing that her insanity would let her, which was kill.
Her urge to kill gave birth to an all new dimension. This dimension was without heaven or hell, a Limbo constructed of hate and grudge. As Yuki said herself, the grudge of those you killed drives you to insanity, which is exactly what happened to Takamine.
LOGIC BEHING THE EXISTENCE
The school isn't like the stable dimension of earth. It's unstable and incomplete. It's held together simply by grudge and hate and sorrow, which is no way to keep balance. Earth is stable because of the equal measurements of Forgiveness and love and joy, which is what gives earth it's complexion. With the Grudge that holds together the school, giving it it's complexion, it makes the school a dreary, horrible place, because there is no other emotion to hold it together.
The constant shifting is caused by the imperfection. It's unlike earth, and is held together by a different means, thus making something made purely of memory, which is hard to control.
Each Child spirit retains it's own version of the school. Sachiko wishes to kill in her own world, which thus gives way for her magic to work in a darker way. The Darkening is controlled by her, and it can only work when ones emotions have been weakened to the point in which Sachiko can control them. Each of the child spirits have suffered the Darkening, giving them the same interest as Sachiko: "to kill in their own world". Thus creating the closed spaces, which shape as "Their own world".
Sachiko's magic is the darkening. When one weakens themself to negative emotions, the spirit of Sachiko (this isn't really Sachiko controlling them. It's them turning into their own "Sachiko", wanting nothing more than to senselssly kill, no matter who it is, a friend, enemy, or even themself.) enters their very being. Since the school already has it's own Sachiko, those who die due to darkening usually do so because of the school not choosing them over the Sachiko they already have, so they are sent into a deeper stage of limbo, not even having a soul, and becoming one with the school, thus giving more energy to Sachiko.
Because this dimension is seperate from earth, there's no heaven or hell, meaning that there is no way for spirits to transcend into anything else. Thus, the limbo keeps them suspended at pain from the moment of death. An eternal limbo. (This didn't come out as good as it did in my head .-. )
Think I might add on to this. I hope I did a pretty good job of explaining the existence of the school. If there's anything you think I should add to my theory, tell me. If you found this helpful because you zoned out when Naho started talking (Which was exactly what I did, orz.) then good. I really have no idea.
I'm also not sure whether I should post this or not. Meh, I guess it can't hurt.