I had a problem with shy students until I ended each lesson with a competition/game that centered on that day´s topic. I even had a little prize (´lil hommie figurines) that I would get out of a gumball machine in the store for 50 cents. The kids went crazy for these (even though they were really rich kids) and displayed their prizes with much pride.
I modeled the class after a very popular game show here in the U.S., "Who Wants to Be A Millionaire" and the class got exciting very fast.
You need to set the tone, too, with some genuine enthusiasm (don´t be fake or you may kill it).
But, I find games and competitions always get even the shyest ones to eagerly participate and try their best.