Teacher Gao needs to take a month off to tend to his ailing mother. The only substitute the mayor can find for him in their rural town is a 13-year-old girl. Already worried about attrition, Gao offers her a bonus if all the students--and not one less--are still there when he gets back. She throws herself into the job like a prison guard, more concerned with locking the students in the classroom (so they can copy lessons they don't understand) than with making them learn. Which makes sense--what more can you expect out of a 13-year-old dropped way over her head into a position of power? She's focused and selfish in her immature way, to the point that, when one student is sent by his family to Beijing to work and help pay for his ailing mother's medicine, her gut instinct is to leave the class behind and retrieve him.
The movie contrasts the city and the country, but focuses on the poverty of both, and Zhang never sugarcoats the issue. In fact, what makes the struggle in both places so devastating is that everybody takes it for granted. We discover the boy is lost in the city, and through him we can see the fate of so many who come to the city to look for work. It's really amazing that Zhang was allowed to show so much poverty before introducing his deus ex machina (in the form of a Today-like morning show) to save the day. And the fact that the ending is more improbable than impossible allows it to make you tear up with joy.