Is this your MathXL class? Your major challenge is the teacher can't see who is between the computer and the screen. If you arrange to call him or her or meet the teacher in person, this is automatically points in your favor. That might not be enough but it is a good first step on your part.
The issue is if someone has your work and you didn't give it to them, how did they get it? From what I am reading, it sounds like everyone can see posted work and this person copied and pasted you assignment (without knowing that you do poorly). That really isn't your problem if everyone can see everyone else's work in an online environment. The accusation is silly at that point, so I must be missing something.
Now if it comes down to this person having to access your account to get your work, then you need to ask your teacher to check with the admin to see if the course admins have reset your password or anything else that you cannot account for such as changing a credit card number or email address.* In the case of some of the Pearson software this is a trivial task, in the case of MathXL specifically, it is slightly harder.
*I have a low opinion of the Pearson Company, so if I start to rant, tell me to stop.
Solfe
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'That was tops! Who's not good at math? I was all, "Four!"' - Finn, Adventure Time.