Exposure to and physical experience of different cultures makes it more difficult to ostracise them. Thinking of actually meeting and casual talking of "other" people. The more and longer, the better. Also, important is the capability of rational thinking, which allows to detect the lies and inconsistencies. Not sure how much of it can be taught at school. Some people are predisposed to blindly follow the authority whatever that authority is, hopefully not malicious. Thinking that maybe an empathy should be important too, but no, quite the opposite. The propaganda feeds on the feelings while the critical thought is its biggest enemy.