Personally? I would be catching a train by myself and staying out of it.

TBH when rubbish like this goes on I call it out. I also believe in principle and TBH I think the way these girls have behaved would have me running for the hills. Moreso than someone who is a little annoying every now and then. I mean what does it say about them realistically? That as soon as one person decides they don't like you they will do this to you. Is that a friendship you really want to nurture? And the cowardly behaviour, and using you as a scapegoat? Ick.

I think Betty is truly the innocent here, and I think the girls are behaving in a way that would at the end of the day make me walk away from the friendships.

However... I also think you should be calling them out on their behaviour. Tell them to stop. It's b!tchy, and very high school. Not just tell them that they should tell Betty... but they should stop acting like children.

This whole situation makes me want to run far away. ICK!