Why is Sissy referred to as "girl number twenty?"
The fact that the students are referred to by numbers rather than called by their names at school reflects the school's severely utilitarian philosophy. Individuality and imagination is not valued, nor is a fanciful name like "Sissy." The children are instead viewed as "pitchers" or "vessels" that are all the same and need to be filled with factual information to become "useful" members of society. Dickens is using an extreme example to assert that children need to be reared and taught as individuals.