How to handle student plagiarism appropriately?
Usually, the way to handle this is to ask both students to come in at the same time. And then in separate rooms, ask them to articulate one of the key points of the essay. It's usually simple to determine who wrote the paper and understood the ideas and who merely copied the text.
However, the key issue doesn't seem to be just whether B copied from A, but whether:
- A allowed B to copy the material willingly (perhaps for remuneration)
- A allowed it to be copied by B accidentally or through neglect (e.g., A left a draft on a public computer); or,
- B copied it by malicious action (B hacked into A's computer).
(There is a fourth possibility: that both B and A used source C and both are plagiarists.)
It would be unfair to punish just B and let A go without proving A's innocence or culpability.
That all being said, untangling this is non-trivial and at this point, I'd declare it above my pay grade and send it up the chain of command to the university committee responsible for student ethics and misconduct (via the instructor on record if you're a teaching assistant).
You go to whoever in your department is responsible for this kind of thing, explain the situation to them and do as they advise. This is a serious matter and you need to be acting in accordance with your department and university's policies, not on the well-meant advice of strangers on the internet.
Speaking as a student lab instructor who recently handled a similar situation, this is the general decision-making process I go through, with concrete examples from the incident with which I was involved.
- Determine if any school policy constrains your actions. For example, in my case it is required that even suspected cases be turned over to the academic misconduct people. This centralised mechanism allows the school to accurately track students who offend in multiple courses, and it offers additional disciplinary options that aren't available to the instructor (e.g. the incident may be noted on all transcripts the student requests).
- If the above step doesn't prevent you from doing so, document the facts of the offence. This may require you to do additional research (e.g. checking the assignment against an online plagiarism detection service), depending on the particulars of the assignment. In my case, the papers were submitted electronically, and I was able to prove that Student A had turned his in barely prior to the posted deadline, but Student B had turned his in several hours late.
- Should the evidence be insufficient to determine who has what level of culpability, it may be necessary to interview the students. This is tricky, because an instructor doesn't have the same powers as the police. For example, depending on jurisdiction, you likely cannot force them to show you their inboxes, or restrict them from communicating with one another before you interview them. I strongly recommend taking steps to protect yourself against the possibility that the student will accuse you of extortion (i.e. they claim you said "I won't report this if you give me money/favours/etc."). You may be allowed, depending on privacy laws, to inform an authority in your department, or to record the conversation. In my case, I informed the professor who taught the associated lectures, and he conducted the interview via email, so neither of us was never alone with the student and there was a written record of the discussion.
- Use your findings to determine what likely happened. Do your best, just like you would when marking the assignment normally, to keep the students' identities at arm's length. That is, don't presume that since Student B is a poorer student, Student A did the work and he copied it. In my case, I concluded that there was no evidence of collaboration before the deadline, and that Student A was likely under the impression that he was free to discuss it with Student B (discussion after an assignment being commonplace and tolerated here, even to the point of emailing work to another student), but Student B misrepresented the work as his own. Student B did not contest this in his email response.
- If there is an office to handle these things at your school, present them with your evidence and, if appropriate, a recommendation. If not, determine what consequences to impose. In borderline cases, it may be appropriate to issue a warning; in clear-cut cases, it can vary quite a bit depending on both the severity of the offence and the local academic culture, from taking a zero on the assignment to failing the course to expulsion. In my case, the assignment was heavily weighted and a zero on it caused Student B to fail the course.
In general, the first priority is to play by the rules of your school and the laws of your jurisdiction.