what lesson is learned by one of the characters in the story

Which particular story are you referring to?

for the story The stolen party by liliana heker

The prejudice that arises from differences in social class and socioeconomic status is the central theme of "The Stolen Party." While Rosaura prefers to believe she has been invited to Luciana's party because the girls are friends, Rosaura's mother insists that she was invited because she is their maid's daughter. Rosaura resists the implication that she and her mother exist on a lower societal rung as a result of her mother's employment as a maid. But despite Rosaura's desire to participate in the party as a regular guest, her presence is questioned by Luciana's cousin. Señora Ines also enforces Rosaura's class position by having Rosaura assist in serving drinks and cake, which she rewards with cash rather than the parting gifts the other children receive. Señora Ines's reluctance to withdraw her offer of money even when Rosaura and her mother are evidently offended proves how deeply notions of class difference have penetrated Ines's understanding of the world, to the extent that even her child's birthday party must remain socially segregated. Ultimately, the experience confirms Rosaura's mother's belief that Rosaura would be treated differently than other children. Despite her attempts to warn her daughter, Rosaura's mother is unable to prevent Rosaura from experiencing the insult of being treated as low-status.