Any instructor can configure Turnitin so that student papers are NOT stored in our search database, and will never be accessible in searches by any institution (including your own).
Our subscription content has over 60 Million articles from over 10,000 sources, and almost all of this material is not available via Google yet is likely to be available through the subscription databases that are maintained by your library.
Turnitin is definitely not a replacement for proper instruction, and nothing will replace the guidance of a motivated instructor−we simply try to provide an electronic tool that can provide a level of data access that cannot be performed without the use of technology. I agree that an instructor may be very familiar with the writing style of a student, and that they may also be familiar with much of the text that exists within a given area of interest, but it is clear that a single human being could not perform an exhaustive search that spans over 8 Billion pages of Internet content and over 60 mllion articles.
I wholeheartedly agree that Turnitin should be used as a learning tool, used by students prior to the submission of a final draft and used to ensure that the student will not have issues with improper citation rather than being used as a post-submission detection tool−we cannot control the instructional methods used by our customers, but I/we are very happy to see that the general move within our customer base (and academia in general) is to use tools such as Turnitin in a proactive, student-facing fashion.
When the Lord finished the world, He pronounced it good. That is what I said about my first work, too. But Time, I tell you, Time takes the confidence out of these incautious early opinions.
Stand and Deliver: A Blog for Faculty 