Trigger Warnings: Essential or extreme?

Trigger warnings are often used to alert people to potentially distressing content online. But some US university students would like to extend those warnings to the classroom out of concern for those who have had traumatic experiences with topics such as rape, sexual assault, suicide and poverty. Critics say these alerts limit freedom of speech, are overused and promote excessive political correctness. So, do "trigger warnings" have a place in society? And if so, should a line be drawn?