Hi there miss Marina,
how old are your students?
I did too come across the same or similar problem a few times, people just don �t have an opinion or have never heard of the topic.
May I suggest a few hints from my experience?
- They must have an opinion on SOMETHING, why don �t you start with very simple issues and dwell on them for a couple of weeks before moving onto something more serious. For instance, they all have their own tastes in food and music, for sure, you could divide them into two groups for and against pop/rap/rock/Britney Spears/pasta/chicken/fish/sushi/jam/ chocolate ice-cream versus strawberey ice-cream... Just for starters, you know. Just to get them going, and once they start perhaps they will discover that they do have an opinion after all.
- Try a few provoking issues - such as "All Americans are fat", "All the sick animals must be killed", "There are only good people in the Chilean government"... and so on. This is probable to generate some indignation.

- An introductory text is good to get people to think, to activate the schemata, so to say. The more shocking the better. It is quite easy to find controversial articles online. I myself have an activity called "Controversial ads", I collect ads from magazines that advertise Cosmetic Surgery, Psychic Future Foretelling, Recruiting Women for an Escort, Coloured Eye Lenses (for fun) et cetera, all having some good stuff and some bad stuff in them, so people can figure out the pros and the cons.
- And lastly, it very much depends on the teacher to lead the students in this. You need to show some emotion on your face - don �t worry about looking silly, look angry when you speak about something bad, look genuinely happy when you speak about something good, drop a joke once in a while, make cheeky statements.
Good luck!