Feisty,
Indiana law is pretty clear about abuse situations, and gives a long list of people who are required to report any incident they see. At the end of the list they give a nice little blanket "and any other person" line that essentially means that anybody in the state that sees a child being abused MUST report it. I believe that failure to report is a felony, while filing a false report is only a misdemeanor. Abuse here covers any crime against a minor.
I've never seen anything of that nature* in my school. I've had sexual predators in my school (I always found out later) but none of them to my knowledge ever victimized one of our children/teenagers. It would be difficult to do so without being seen, given the floor plan. They can't get access to a victim, and its a very high traffic area.
If it was a situation outside of the school I would likely intervene, as well. If I saw one of our fourteen year old girls out with a twenty five year old, and I saw a public display of affection that was overtly sexual...I'd call the cops. She'd hate me...probably forever...but integrity and the law demands it.
Note here that my impulse would be to choke the guy. There is nothing wrong with an impulse as long as you don't act on negative ones. I would fantasize about choking him, and that would have to suffice. It accomplishes nothing in such a situation if a perfectly good witness suddenly becomes just another defendant.
*Specifically I've never seen sexual abuse in my school. I've seen physical abuse and reported it to Child Protective Services. A woman picked her misbehaving four year old off the floor by her hair and carried her that way to the car. The girl's feet never touched the ground for a distance of about twenty or thirty feet. It is one of the few times in my life I have ever been left speechless. I lost a lot of sleep over that.
Regards,
Steve