The Mother (some spoilers here)

The mother was obviously disturbed by her husband's treatment of her son, yet she only intervened twice. First, she sold her own possessions to send him away to boarding school. Second, at the very end of the movie, she verbally objected to the step-father's planned beating of her son. The rest of the time she retreated to her piano to escape from the sound of her son being whipped. (I found the picture of the "woman with back turned" to be completly unnecessary.)

Presumably she feared her husband. At the end of the movie, the son cautions the step-father not to get the police involved, as they will then find out "what you have done to us." So maybe he abused the mother as well as the son?

Or perhaps the mother was so economically dependent on the husband that she dared not intervene on her son's behalf? I don't recall the movie telling what the original husband did for a living or explaining why the mother was now married to a waiter.

In spite of her long-standing failure to protect him, the son obviously loved the mother and his stoic heroism at school seemed based largely on his promise to her that he would not get himself expelled.

I also found no explanation in the film for why the mother finally did come to his defense in that last scene. Perhaps after learning ( from the lawyer probably) of what her son had endured in order to complete the school year, she finally felt she owed him her protection.