Plagiarism is a serious offence wherever it is committed. Plagiarism is often committed in school, when students may use it as a devious means to avoid writing original work. Today, plagiarism is easy to detect because of plagiarism-catching software. If you are accused of plagiarism, but did not intentionally copy someone's work, you'll be required to defend your work to a teacher and/or department head.
Ask the teacher to explain to you how and where in your assignment she believes you committed plagiarism. The teacher may indicate the entire essay or simply one paragraph or section. If you did not intentionally commit plagiarism, arrange a time and location to meet with the teacher to refute this claim. Because plagiarism is taken seriously, the teacher will likely have given you a failing grade on your assignment. If you can successfully refute the plagiarism claim, the paper may be regraded.
Assemble all of your reference material and rough drafts of your work. If you did not intentionally commit plagiarism, it is most likely your teacher will have flagged only one or two areas of your essay. An accusation of plagiarism may have been caused by your failure to correctly cite a reference in the footnotes or bibliography, omitting quotation marks around a statement that is not your own or writing a statement that is too similar to one found in a book or online. To defend yourself, it is important to prove you did not intentionally commit plagiarism.
Explain that your footnotes and bibliography were otherwise error-free if the plagiarism claim arose from your failure to correctly cite a reference. Stress that if you have correctly cited every other reference in the assignment, there is no reason you would intentionally attempt to mislead the teacher by failing to cite another reference. If you can recall a legitimate reason that you may have made a mistake or omission in your citation work, explain that to the teacher.
Explain that you somehow managed to delete the quotations around a piece of quoted information, if applicable. To do so, show the teacher any rough drafts of your work that support your claim. Because it's possible to mistakenly delete quotation marks during your final editing process, a previous draft of your work may have the correct quotation marks in place.
Explain that you have read several books on your topic and that an example of plagiarism is likely coincidental and based on you unknowingly using similar terms to a piece of published work. The "coincidence" argument is weak if there are several instances of plagiarism in your work, but if there is only one, the teacher may believe you.
Explain, if applicable, that you have never been accused of plagiarism in the teacher's class or any other class. This defence alone will not suffice to exonerate you, but in cooperation with other excuses, your teacher may believe you.
If you have intentionally committed plagiarism, consider owning up to your mistake and asking for a second chance. If you repeatedly deny it, and are found to be lying, your punishment could be more severe.