Probability is not proof.
Proof is tautological... either you have it or you don't.
The professor knows people cheated because students have confessed that there has been cheating, but he can't prove that they cheated, even with statistical analysis.
No dean will flunk a student who he suspects MIGHT have been cheating. That's simply a lawsuit waiting to happen.
The problem here is with the professor. A 400 level class that has a freaking scan tron test? Just how lazy is this guy? If the students simply memorized the answers from the test bank, then they have fulfilled the objectives of the test, because a multiple choice test simply requires you to choose the correct answer from a list.
Know what kind of test you can't cheat on?
A test that requires you to answer the question and explain how you came about the answer.