If you want to learn about people in a group or classroom or anywhere else, you cannot make rules which restrict language, attitudes, or opinions. The most you can do is to forbid physical attacks. Verbal attacks are fine if they teach something. Professors may or may not be able to handle this. They used to be ladies and gentlemen when they wore coats and ties and dresses. They were not accustomed to free speech that could be unpleasant. But we have come a long way from those days. Nothing is really taboo any more; we can regret this, but it is a fact. People can be urged to give others a chance to express themselves without interrupting charges of this bad or that bad attitude, but you cannot purge unpleasant views. Some may even find it possible to be negative in a polite way, but I don't expect that very often.