Could confirmation bias be behind this false accusation of plagiarism?

As some of you may already know, I played a very big part in raising my sisters. My youngest sister, who is now a senior in highschool, wants me to attend a meeting with her teacher and principal to discuss her punishment for plagiarizing a paper in AP English.
Of course, the problem is that she didn’t plagiarize.

Apparently, what happened is that another student did plagiarize the same assignment, and had copied large portions of text from Wikipedia. My sister’s paper stood out because it was “too good,” and her teacher found, from what we understand, 2 lines that looked similar to another paper that can be found on the internet. She claims that only a few words have been changed in the lines, one of which was to add the word “amalgamated.” For whatever reason, she didn’t believe that my sister even knew the word amalgamated (which is ridiculous, because my sister is a very bright kid and an excellent student.)
Anyhow, my sister brought in her sources, but this teacher insists that she does not believe that it wasn’t copied. Apparently this teacher has already discussed the situation with other senior teachers and the principal… which I find upsetting, because I don’t think it is fair that my sister’s other teachers may now be suspicious of her work. The teacher also told my sister that if she brought this situation to a meeting, that she would go over her paper with a fine tooth comb and find anything and everything she possibly could that might be wrong with it (which sounds like a threat, to me, but that is an aside.)

Anyhow, I’m not saying that I believe this teacher has malicious intent… but could this be confirmation bias at play? Could it be that she found something in my sister’s paper simply because she was looking for something? I can’t imagine that if you compare the wording in student papers written on similar topics that you won’t find countless examples where certain things may sound alike. There are only so many ways to say certain things. This woman has no reason to doubt my sister’s ability to write a good paper, so the whole thing seems like it is out of left field.

We haven’t actually seen the paper she is comparing it to, and my sister has no idea what part of the paper is actually being called into question (which is one of the big reasons that I believe she is telling the truth.) Does she have any line of defense against a school that seems to have already deemed her guilty? If she fails this class, which the teacher says she will, it will ruin her 4.0 GPA. So, obviously aside from having her reputation tainted, she has huge concerns about her grades.

Sorry, this was really long. Thanks to anyone who reads it all.