The teachers don’t know the specific questions, they just know the type of question and the previous questions.
Professional boards are done the same way. The prep courses give out old tests, the students answer all the questions on all the old tests and study them. Almost all the questions can be found on one of the old tests.
If they really wanted to test the students and not the teachers, they would be using something like the IOWA test that you can’t prepare for.
When I was in med school, we had old tests to use as a learning tool. They mixed up and wrote entirely new questions for the current class. It was very helpful to learn what they were looking for, and not waste our precious time on subjects that weren’t going to be asked anyway. We did have a lazy pathology prof who wouldn’t let us have old tests, because he used the exact same questions every year. He let us look at the test after it was over, so we’d know what questions we missed and could bone up on weak areas. But it was posted on a bulletin board, behind locked glass, lest anyone make a copy.