I'm considering the teenager here when I say it's probably best if you go to the teen first and talk to her. Hint around inappropriate relationships or something and see if she brings it up herself. Maybe there is apart of the teen that is dying to let this out but is scared too.

Just because your suspicious doesn't mean it's actually happening. This is a very serious accusation that could destroy this teachers life. I'd be sure to validate your vibes on this one.