When I was in high school, I fantasized about many of my teachers often, some of whom weren't even all that hot. In those cases I guess it was the whole older woman/authority figure angle more than anything else.

My REAL fantasy, though, was the school secretary. I don't know what her actual age was, but she was relatively young for the position and loved me to death. I thought she was hot enough in the first place, but one day I went to the office to get some copies made for a class, and when she leaned over the front counter, I got a look right down her shirt and she wasn't wearing a bra! That equaled instant hard-on for a 13-ish-year-old. I think I had to untuck my shirt a bit before walking out. That moment provided whacking material for years.