I chose the wrong teaching method. Instead of dividing the class into four groups according to language competence levels and giving each group tasks according to their abilities, I gave one task for the whole class.
What happened and why?
As a result of such a decision, some students could not cope with the task at all while some other students completed it quickly and started chatting to each other and disturbing others. It happened because I did not know my students' English language level.
What did I learn?
I learned that tasks should be given according to students' ability and language competence. They should be challenging, however should not be too difficult. So now, the first time I meet my students, I give them a diagnostic ESL test. Results of the test enable me to arrange students into groups according to language levels.