I don't think this happens all that often. Not as much as this recent spat of posts would suggest anyway.
A teacher at my school had a horrible attendance for two years because he was always in court &/or being required to stay off due to the fact that he had boned a female student at his previous school. She was 17 too, so it was legal here, just he broke one of those rules by fucking someone under his care.
There were rumours that a guy had boned a (female) teacher at my school but while he 'confirmed' it & it seemed very plausible, it was never proven to be true. (not that anybody reported it or nothing)
As for teacher on teacher, you know how schools have their own little tales about shit that had happened there before? Well it was always said that these two teachers were lovers & pupils would constantly bring it up to take the piss out of them & then, low & behold, the other month they just got married - after denying it for like 5 years!! (to both pupils & teachers, although I think the teachers had it confirmed a few months before they were married)