How to deal with a girl who's threatening to beat me up?

So this girl at my school messaged me calling me a bitch and she's mad at me. She said that she's mad because I supposedly messaged her boyfriend. Which at first I was confused at what she was talking about but then I realized that me and her boyfriend were both sent a mass message so when I replied, it probably showed up in his messages. But she won't listen to me and she's even threatening to beat me up... honestly she's acting stupid about this anyway. Even if I did intentionally message her boyfriend (it was just a thumbs up emoji) it's sad that she's getting so defensive over another girl messaging him a totally innocent message. Apparently he's not allowed to have female friends.. but whatever that's not the point. How do I tell her to back off without getting her angrier?
How to deal with a girl who's threatening to beat me up?
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