Ivan,
Thank you for posing this question. I have done some research to see what resources on this topic might be available for educators. Though the concept of trigger warnings has come up in the media over the past month, I actually wasn't able to find any formal discussion or study of it appearing in a reliable medical, scientific or pedagogical source.
It appears as though many educators -- at least in higher education, according to this New York Times story -- are improvising their responses to requests for trigger warnings. This Huffington Post piece has a list of similar stories, some of them critical. You may have seen these links already, and they may or may not be useful to you.
It would be great if other educators might share their thoughts on this topic. I wonder: Are trigger warnings inherently "trauma-informed," or is there a way in which trauma-informed practices can most effectively shape the response to the concern of re-traumatizing students in the classroom?
Rebecca