Thanks to all of you for your answers.
I like becki's idea too...and that's pretty much what i actually did.Thing is that the pieces of advice came later than the date when i had to teach. It went fine and in case somebody will use this again i should mention i started with dividing students: on one side those born on an odd day , on the other side those born on an even day,asked one group to prepare for a test while the others could do whatever they wanted to:)...then asked them how do they feel about it? Is it fair or unfair?And the students could guess the topic of the lesson.Then -brainstorming on discrimination and on types of discrimination ;the "hot" picked topics were discrimination against age, against gender and against race...introduced new vocabulary related to the main topic, talked about the bilogical myth of race, and had as group activity racism- learnt or taught? where they came up with interesting ideas and proved they actually had more knowledge than i expected them to :) As i said , everything went fine except that they seemed to not feel discriminated against , they were aware that discrimination exists in society but they have strong personalities and simply don't care about what others might think about them...a lesson learnt for me too!