If the teacher will not budge, I would go to the principal. The principal is the next step. And after that the superintendent, then the board (the school board)
I would not waste time/energy on the teacher/assistant principal if you feel that is not going to change the circumstances.
I agree with you...what you describe? The using a quote with the intent to reword? Frankly, that is common. So I can understand how this happened. And it should not ruin his HS record. Period.
That said, I also understand the teaches point of view. Plagiarism is a serious concern at ALL levels of school...much more so today then my days of school prior to the ipad and internet.
So giving a zero on the assignment? That is tough to fight. Zero for the entire class? That is much more difficult to swallow.
I would approach the principal. I would be reasonable and acknowledge the misconduct
. THen talk about a reasonable solution.
If the principal will not be reasonable? I would probably go straight to the board...the superintendent will most likely be read into this already...so if the principal will not correct this, I would go straight to the school board. They will have a public meeting regularly (typically once per month)...show up and state your case...best to put in to a letter and send to the board then show up and speak in person at the meeting.
I doubt you need to go this far, but if you do the school board has the power to remedy this.
You could go further if the school board refuses to address this...you can actually sue to force this issue. But what you describe, if you present this reasonably, I bet that is not needed.
Let me know if you have more questions, happy to help if you can