Well, I'd guess the only potential legal issue is making sure that absolutely everything, down to even the minutest detail, is in fact true. Embellishing anything could find it all blowing up in your face pretty quickly in a courtroom.
Your position in selling the story also matters. I take it you're the second party in the affair and wish to "expose" your relationship with one of your teachers? If not, and you're actually just another teacher who has noticed a colleague gettin' jiggy with the young'uns then you're in no position to approach the media. Rather, you should be approaching the local authority or school board.