Happens a lot by the sounds of things. At my school there used to be a lot of sixth-form / teacher shenanigans (pretty much 100% male teacher / female pupil) but this was stomped out by the time I left. All kept very discrete at the time, might have been 'frowned' upon but was certainly tolerated - those I knew about the teacher was usually in their early twenties.
We had a music teacher who was known to be a bit 'dodgy' - would invite boys back to his for 'recitals' (usually involving alcohol and cigarettes) but how he got away with it for so long is a mystery. I think he left 'under a cloud'. There was the PE teacher with a penchant for spraying a cold water hose on boys backs whilst they swam in the pool, he was sacked for some sort of improper conduct too.
The most infamous example was when we had a new, young science teacher join the school. Looked just like Mr Muscle from the cleaning ads. Was very 'right-on' and involved in sports clubs etc, big in the youth-church locally too; they all thought he was fantastic. Which I'm sure he was until he invited a bunch of lads back to his after football. He then proceeded to ply a couple of them with alcohol, get completely plastered himself and then, in a state of undress, make a series of indecent propositions to one particular lad. Who, being resolutely straight and built like the proverbial outhouse with a temper, proceeded to lay him out flat before calling his Dad, followed by the police.
We didn't see him again...