<p>At my university, and I believe at most others as well, there is a "Code of Teaching Responsibility" that defines the teacher's obligations in certain areas. One of these concerns returning papers and tests; at my university we are required to keep them for up to one year and to return them to the students upon request. Who enforces this kind of thing? Either the Department chair, as the officer of first resort, or a university Ombudsman who makes a polite intervention to resolve the dispute. My guess is that the OP's college has something akin to this system. In addition, depending on how that paper was submitted, it may well be archived either in the e-mail system or in the course or class-management system. For example, at my university students submit their papers into an on-line "drop-box" set up for the given paper and class. So if that system is in place, the paper probably hasn't yet been deleted or purged.</p>
<p>There is nothing wrong with resorting to one of the options that I mentioned above. The OP should either go to the department chair or the ombudsman. And another resource is the students' academic advisor, who may also be able to intervene constructively or advise how best to proceed.</p>