As a student I remember never getting an attractive female teacher. Except once, a one day substitute.
I think fantasies over an attractive teacher is normal. If those fantasies became real, surely could be fun, but could be negative consequences in the long term. I think it depends on the situation whether it would be harmful or not in the long run ... case by case basis.
I think many of us have fantasized about an attractive adult as a teenager, I had crushes on rock stars for example.. lol but that doesn't mean if I actually had a sexual affair with them it would have been fun..
When I was a teenager it was still common for teenagers, teenage girls & gay boys in particular, to get groomed by adults and to date people who were much older.. I myself had a long distance relationship when I was 15 with a 23 year old, and it was not a great experience let's just put it that way. Everyone I know who experienced this was traumatized for life. It was glamorized in the media, too.
When it's happening, it feels ''fun'' because you're getting attention from someone older. Adults who do this usually pick lonely or troubled kids.. Then you realize it's not that fun and that a lot of things are wrong and abusive about it.. And you remember years later a lot of details that were really messed up about it.