I'm guessing it is a more emotional thing than a , "I wanna bang a little kid" thing. Women want to be wanted. Who wants more than a horny 17 year old, especially with a semi-attractive teacher nearby.
I had the hots for my Spanish teacher in 7th grade. I saw her after I graduated and was horrified at how ordinary this woman that I lusted after looked. Giant schnoz, no boobs, big butt, but she was the only female teacher I had under 50. As Hannibal Lector taught us, "We covet what we see".