It was part of the new season opener, the deputy head kissing a girl he's had back to his goodbye (met in a club) and leaving her in his bed with a cuppa as he goes to work.
Girl then turns up as one of the new pupils and the new headmisstress' daughter! Cue the whole "why didn't you tell me how old you were" conversations, do I tell the head dilemma etc...
He quit but then another schoolgirl went missing and he got caught up in the search and decided to stay though has not confessed to the head as yet.
Not had the full story about the daughter's disappearance as yet, moreso angst from the family and a few flashback/imaginary scenes. Head had a moment when pupil went missing due to daughter's disappearance and so the deputy head (the one who slept with the other daughter) was leant on for support (he helped out after two other students went missing at the end of the last series) hence part of the reason he ended up staying.
Hope that makes sense!