You've probably seen this, but I hadn't before another teacher linked it to me. As a teacher of teenagers, we spend a lot of time trying to empower them to step up in these situations. This was filmed on the campus of where I did my undergrad and grad studies. It doesn't surprise me at all, sadly.
The main lesson is, of course, that small females are the biggest badasses around.
https://www.youtube.com/watch?v=EisZTB4ZQxY&list=PL29CD4FDCF451DEFD (https://www.youtube.com/watch?v=EisZTB4ZQxY&list=PL29CD4FDCF451DEFD)
I've actually been in that situation, and that small girl was very brave. It's fairly easy for me to throw my bulk around and get between 2 guys, but that girl had scrap.