I used to allow students to keep their tests after they are graded, but now I take the back up. I do this in anticipation of a student like one who came by today. He indicated that he studied and studied but still didn't do well on the test. Then, I took out his test and noted question after question which was identical to questions on homeworks, and I told students directly that those questions would appear on the test almost exactly as they appeared in the homework.
He surrendered, admitting he did not study very hard.
Teachers, keep your tests, they are good ammunition.