A girl in my class always creates a misunderstanding between me and my best friend. My friend trusts her blindly and fights with me for nothing. I can’t do without talking to my friend and I find this situation unbearable.
Looks like you feel really helpless and annoyed when your best friend blindly trusts the other girl in your class, and fights with you. If it takes such little effort to make her fight with you, then it is obvious that she doesn’t exactly trust you — and trust is the foundation of friendship. In that case, it seems that you consider her your best friend more than she considers you her best friend.
If this friendship means so much to you, then it is important to have a frank conversation with her and understand what triggers her to fight with you. Let her know that it hurts you when she does this. If she does care about you, then mutually decide what action either of you can take if someone tries to create misunderstandings between you both — perhaps you can check with each other before drawing conclusions or taking action. If she doesn’t agree to participate, or change her responses, then I’m afraid you better off finding new friends who trust you as much as you trust them!