I was a teacher in a previous life and I would want to know if someone had swiped the mock - I wouldn’t mind if it was totally anonymised and if a note was left on my desk, even typed out, because I would be inclined to trust my students in that situation and understand why they didn’t want to land themselves in trouble with their peers. I would just really appreciate knowing so I could select and print a different paper. I don’t even know that I personally would call my class out on it - just changing the paper would be enough for me; but I’d let my HOD know.
That said, the teacher will 100% know if a student suddenly performs way above their usual parameters and it will raise questions, so something will get found out one way or another, even if it isn’t the full truth.