A year or two ago, on a private e-mail list, I stated that I did not think there is anything wrong with collecting old tests that a teacher has given and studying from them, even if the teacher is known to regive old tests. Others, however, considered it unethical.
R. Yisroel Belsky discusses this in a column for Torah.org:
It is hard for me to say that it's ossur (prohibited) or that it's immoral. It's a higher level of morality not to do it, and study. But it's definitely not something that you could even taint by saying it's ossur or by saying it's immoral, because it does lead to a good knowledge of the material. It provides a pretty decent review.
If there are only one or two tests, then the teacher is a fool. The students will just copy the test and not learn anything. But if there are five, six or more tests, a teacher worked very hard for several years making the tests, and if you go through all the teacher's tests, you'll probably learn a lot.