This answer contains spoilers… (view spoiler)[I agree parts of it were confusing. Sader was a Seer and therefore able to temporarily host the good schoolmaster's spirit just long enough for the battle. And Sophie/Agatha disappeared (and saved Sophie) because true love's kiss (friendship kiss, though the book, I think, is intentionally vague about it). But I don't quite get the kiss part because the evil headmaster said Sophie was his true love so why didn't their kiss do anything? (hide spoiler)]
Ray-ray Okay, so Sophie was revived and now for some reason there now back in the home town. Pro. Sader was a ghost-host. AS in he could host a ghost in his body and the ghost would have complete control. The School Master's DEAD brother went into Sader to figh the School Master.