Why does Clarisse consider herself responsible?
Most of the children Clarisse's age are mindless drones. This is how the education system works in this society. They give children TV classes, transcription (copying) history classes, and sports activities. The kids are brainwashed, minds are numbed, and their only outlet beyond the sporting activities is to bully and beat each other up. Clarisse is unlike these kids in every way. She doesn't simply go with the flow and accept this kind of education. She asks questions. She is disciplined. She wonders why people hurt each other. She has no friends her age because she won't participate in the bullying, drone-like type of mentality the other children exhibit.
In this society, most people and most children do not think for themselves. They are brainwashed. They do what they've been taught and and they do what the authorities tell them to. Since they do not think for themselves, they are not really in control of their own behaviors. Since they don't control their own behaviors, they are not responsible for their own behaviors.
But Clarisse does think for herself. Therefore, she takes responsibility for what she says and does. Clarisse adds that she was spanked and therefore has learned what is right and wrong. But it goes deeper. Unlike the children and adults in this society, Clarisse thinks and acts with her own free will. Being free to choose, she is responsible for her words, thoughts, and actions. She is an existentialist in this way.