This sort of thing has being going on since the beginning of schools I guess. I know it was happening when I was in school 100 years ago (ok, maybe 25 or 30) - there were three female teachers at my high school that were known to be involved with students and one security guard (male). Back then, the teacher just got transfered or fired or nothing. You didn't really hear about it in the news. Another ancient friend of mine was pursued by her assistant high school principal (he'd call her out of class to "talk", write her love notes, etc...) - her parents found out and had words with him, but nothing was ever done beyond that.
I think that the profession itself attracts these types. The majoriity of teachers are good people who want to help students learn but some of them choose the profession because they are more comfortable with teens than adults...they haven't matured enough themselves. I think it's these who prey on the young teens.