If the test was multiple choice, perhaps two or more people had the exact same answers for every question. That would be a plausibly innocent scenario only if the students accused of cheating answered nearly every question correctly and shared a couple common wrong answers because they studied together outside of class and were therefore susceptible to making the same error. The group of students with identical answers would need to be large enough as to not restrict suspicion to an otherwise small number of students with matching answers. That's a low likelihood (say around 20%), but conceivable.
If, however, a group of students shared a lot of matching wrong answers, your teacher would know with 99.9% certainty that there was some cheating. She wouldn't, however, necessarily know who copied off of whom. Furthermore, if the matching wrong answers correspond to "right" answers from a different exam, then your teacher would have reason to suspect that someone in the class stole the answer sheet and supplied it to classmates with the same wrong answers. But the teacher wouldn't necessarily know who stole the answer sheet, and/or who distributed the answer sheet, without students in the class coming forward.
If this were an essay-based exam, and two or more students submitted the exact same answers word for word, then it's extremely clear that there was cheating because the likelihood of two people phrasing a long answer the exact same way is virtually non-existent. But again, there would probably have to be a large group of students submitting these identical answers for your teacher to address the entire class instead of the smaller group of two to three co-conspirators.
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