@ kuma , I wouldn't say they're overpowered. Every character has some kind of weakness in this manga. =/ Even Kogarashi is weak to transformation techniques but is otherwise pretty OP by himself.
"Adults don't believe in ghosts at all!"
"We're the only ones who can save her!"
[Mic-chan and Mao-chan successfully channel the power of The Goonies.]
@DaylightBlue the why: she wants to enjoy living as a student because she never had that experience. The how: the principal of the school is one of her acquaintance that knows her secret, so he can enroll her.