I used to manage the student disciplinary process for a Faculty of a college, including assessment malpractice.
Our college issued clear guidance to students to help them understand how to avoid plagiarism and promote 'academic integrity'.
Have a check of yours.
Our college had a clear policy published about what constitutes assessment malpractice and gave advice to students how to defend themselves and what support they could seek, how to appeal.
Were you able to get a copy of the Turnitin report? That could help with your appeal as you would be able to dispute the findings of this software.
Remember that plagiarism isn't just about directly copying another students assessment or failing to cite sources. A student may also not be aware that they plagiarised because it was not an intentional act.
It can also be the result of the student failing to foreground their own voice and own style, and heavily paraphrase the content of acknowledged sources outside of cited direct quotes, when the assignment is a patch work of direct quotes surrounded by paraphrasing.
Here is some guidance from different university sources. This isn't aimed directly at you, just that I want to shine a light on this area :-
"Only include other people's ideas within your work when they support your argument or to illustrate a point and ensure you reference them.
Try and think in terms of 'ideas' and not quotations or paraphrases; if something is not your idea then whose is it?
Use direct quotations only when absolutely necessary, focus on ideas instead."
"It is, for example, possible to cite a source in the separate bibliography and still commit plagiarism by then incorporating a significant amount of un-attributed material taken directly or indirectly (through paraphrasing) from that source into the body of the assignment."
" Avoiding plagiarism is not simply a matter of making sure your references are all correct, or changing enough words so the examiner will not notice your paraphrase; it is about deploying your academic skills to make your work as good as it can be.
It is your responsibility to avoid close paraphrasing.... It is better to write a brief summary of the author’s overall argument in your own words, indicating that you are doing so, than to paraphrase particular sections of his or her writing. This will ensure you have a genuine grasp of the argument and will avoid the difficulty of paraphrasing without plagiarising."