So if you had a 17 year old daughter getting banged by her high school teacher, you'd be okay with it.
A gangbang with only 2 people? Make group sex great again my friend! Having no kids, I shudder at the thought of ANY potential daughter of mine of whatever age gangbanging anyone or anything. I shudder at the thought of any potential son of mine gangbanging anyone or anything as well. But just because something is distasteful to you doesn't automatically make it criminal to me. It IS illegal based on the existing statutes - not arguing that. I'm just rarely ready to broad-brush anyone's sexual gymnastics into arbitrary categories based on someone's moral objections.
Should you point to an overt abuse of power (NOT merely being a teach in the same school) I can reconsider this case.
And teachers should not sex up their students and vice versa. It's unseemly but not criminal in many cases of 100% consenting adults.