She probably didn't know, or didn't care. Sexual urges are powerful things. I know when I was 12 I had a teacher who I became very close to. By the time I was 14 she was taking me home after school. She used to put her hand on my leg all the time. Nobody said anything about it back then because we all knew teachers who were having sex with students. I remember in middle school a popular boy coming to school on monday describing his math teachers tramp stamp that he saw when he went to her house that weekend and had sex with her.
She never got in trouble. I mean it was not even uncommon. It was a joke to us, see if you could bang a teacher. I never did because that teacher who took me home was more like a 2nd mom to me than any kind of romantic thing, but it could have turned that way easily had I let it. I mean she used to bring me into her classroom during lunch and lock the door and turn off the light and have us sit where nobody walking by could see us and we spent a lot of time alone like that. That happened for almost 2 straight years.
This stuff isn't happening more now, it's being REPORTED more now.
I remember well as recently as the 80s or so music videos called "Hot For Teacher." We've become decidely more repressive about sex in just many of our own lifetimes. I'm not sure why.
This particular incident seemed to involve a long-standing relationship. Article said 8 or 9 months. So this wasn't a momentary lapse of judgement but rather a full on relationship. Illegal as all get out, but not something tawdry and illicit.
Prosecute her? Have to, it's the law and there's no provisions for judicial discretion. If there were, I'd say have the 12yo interviewed by a pscyhologist. If no signs of abuse or coercion are in evidence, throw out all charges unless one for being stupid exists. I'll check.