Although you haven't stated the critical details, I am deducing that this is a private secondary school. If that is the case, you have very few enforceable rights.
Check your enrollment contract -- it will almost inevitably state that the school has the authority and discretion to deviate from the usual procedures whenever the school determines that deviation is necessary or appropriate.
In all events, you cannot sue a teacher because you disagree with a grade, nor can you sue a teacher for failing to follow the school's administrative procedures.
Theoretically speaking, you can sue the school for failing to follow its administrative procedures, but these lawsuits are expensive; they take years to see through; they are not handled on contingency or pro bono basis; and they are almost never successful.
If you are accused of academic dishonesty, you need to play defense here. Ask your folks to get you a lawyer. Depending on the strength of the evidence and your proximity to college applications, the most worthy result is often a negotiated withdrawal with an agreement for confidentiality as to the reasons.
But of course, the specifics of your case will drive the nature of your objective.
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