First of all, there is nothing wrong with using old tests to study. This is an acceptable practice, and, at SFU, our Computing Science/Math Student Societies had a Filing Cabinet of old tests that we could photocopy to study from.
The first time this happened to me, it was Physics 12, 100 Mile House - Some of the students had siblings who had taken physics from the instructor. We got the test ahead of time. Took the test - which was identical to one from years gone past. Did well in the test. Teacher Found out, got pissed off with us - I felt like crap.
I ran into this precise situation in my Second Year Data Structures class in '88 in Coquitlam College - there were about 120 students, and we were studying from old test. Only problem is the instructor had used the exact same test at BCIT (another technical college) a week earlier, and so we had a copy of it (unbeknownst to us) - We started taking the test - I realized by question number 2 that I had seen this test, and stood up and approached the instructor letting him know that I had already seen this (I had learned my lesson from High School) - Ironically, he asked me to just try my best, and seemed to hope that I hadn't memorized all the answers. He later asked me to become his Teaching Assistant and Lab Instructor. Honesty pays off. :-)
The final time was at SFU, the instructor had accidentally printed out her new test for students to study from - I picked them up, saw the date, and immediately told her _before_ we took the test. That time she just re-wrote the test and we took it a week later. Once again - zero doubt on how to handle the situation. (Clearly, my high school "cheating" experience was had been a formative experience.)
Net-Net - The cheating is NOT in studying from old tests, it's from realizing you've already seen the test and not letting the instructor know. I didn't realize this when I was in High School, but had matured enough ethically that by 2nd year of college to not even consider taking the test without telling the instructor I had seen the questions.
I think it says very, very sad things about students these days that not ONE of the the 200 had the moral fiber to stand up and say "Hey - I've seen this exam already."