If you're accused of self-plagiarism, and if you didn't do it intentionally, there's two unique considerations involved. In most schools, self-plagiarism will be an offense under a school's academic misconduct policies. At most schools also, even if you plagiarized, but did not do so intentionally, it would be considered an academic misconduct offense.
It depends on the specific circumstances at hand in terms of how your self-plagiarism came about, basically reusing your own work for another purpose. It depends again on the specific circumstances and facts as to whether or not you actually would be culpable under your school's code of conduct, so it certainly would have to be considered. You cannot depend on the school to naturally do the right thing or to be receptive to a student's claim that they in fact did no wrong when in fact that may actually be the case that they didn't do anything in proper.
Having an experienced attorney advisor will be your best ally. They can help you understand and navigate the process and can help you take the best steps forward when responding to an allegation of self-plagiarism.