We are concerned about the ethics of your question, and that’s why people are telling you to leave them alone. It is highly unethical to answer your question. It’s like answering a question where someone asks how to assassinate the President.
If you are going to be an ethical person, then you have to frame this in terms of your concerns. You should talk to each member of the couple, and tell them you are concerned they are bad for each other, and ask them to stop seeing each other.
You must be honest and straight-forward about this. You must be prepared to explain your concerns. You must offer reasons that will help them understand your concerns.
There is no sure-fire way to do this, but you can be damn sure that if you are sneaky about it, they will hate you when they find out and your reputation will be shit. Doesn’t matter that all your other friends think this couple doesn’t belong together. If they think so, they should tell the couple, too.
Anyway, if you are serious about meddling in someone else’s relationship about which you know nothing (even though you think you know everything), then be honest about it. Go directly to the people involved.
My daughter is 14. She sees her friends going through this kind of thing, but she is wise enough to stay out of it. The other way to break them up is to wait. Relationships of people aged 13 typically is no longer than a couple of months, if that. Of course, some will last longer, and others will be shorter, but people your age are too inexperienced and too immature to be able to keep a relationship together, on average.
So your best course of action is to wait. Come back here in six months and tell me they are still together, and I’ll send you a chocolate bar. Or a gift box from Lush.