I actually don't understand why she is angry... Could you explain why?
She is angry here because they gave the Chinese kid a math book in the painting. In doing this, they are stereotyping Chinese kids by assuming that all of them are good at math, which is a common stereotype in the West. Even though it may be perceived as a "good stereotype", it is still a stereotype and is racist. Therefore, she is angry when she sees this on the mural.