You ask two questions:
1. The archiving policy of student (examinations) work
This is not as simple as in the paper era. Try to express the school policy in more detail. For example, what does "keep student's work for 2 years and then can discard" mean? Do you start counting once the student leaves the organization? Is his account blocked thereafter? I mean there is no point in archiving something, if it keeps on changing.
2. Past assignments are not available
This is in the live server? Something is wrong then. The uploaded documents are normally there unless you do something drastic, like purging the user, resetting the course, etc. Sorry, I don't know anything about the Turntin side.