That happened with a girl I graduated with and a new teacher we had our senior year. She would always be in his class hanging around so it was pretty obvious there was something going on. I think they started dating “officially” when she turned 18 and got married a few years after we graduated.At my HS, one of the Driver's Ed instructors was engaged to a Junior. He was in his 30s at the time.
Not sure if they ever got married, but I remember her wearing the engagement ring at school.