This actually happened at my high school.
The teacher never got fired because the school didn't find out. But a friend of mine went to a party and when she got there, a dude we went to school with was there with their mutual teacher as a couple. The host of the party was someone that they didn't realize they both knew, and nobody knew he was dating his teacher.
She was shocked but never reported it because he was legal at the time (I know it still not allowed), they were in sixth form so they were 18, the teacher was in her 30's.