In an educational environment, trigger warnings should not be an issue. It is commonly accepted (or I think it should be) that these environments are a place for exploring topics outside of one's comfort/area of knowledge. It is about education, not about reacting personally to the content being taught or shown.
Sometimes rationality is hard.
That's fallacious. There's no inherent benefit to making the challenge a surprise - a warning is just that, a warning. Not a way of saying "leave if you can't deal with rape images".