Checking Plagiarism among student's work (PDF documents)
As far as I remember (it's about 20 months since I last used Turnitin), the only way Turnitin can check for plagiarism in which the source is another student assignment is if that other student assignment has been placed in the repository.  So you can either choose to keep your students' assignments out of the repository, or check for student-to-student copying, but not both.