It is important for this story that the protagonist be a women. Besides the class structure that existed - and still exists to an extent - in England, there is a structure and a heirarchy based on gender. In the early 20th century, a young woman was more at the whim of her family and her elders than a young man. She was expected to be concerned with affairs of the home, and not what was happening in the outside world. In addition, she was expected to obey. We see Laura being pressured by her family, and we see her obey. She had no other option.
In addition to being able to stand up for himself, a young man would be more expected to be concerned with the lives of the laborer. This goes back to the fact that women were expected to be concerned with the home. Therefore, they were more isolated from the different classes then men were. Because men were allowed to socialize more, and were expected to interact with a variety of people for business purposes (women had no business purpose), it would have been more accepted for a young man to be concerned about the accident than a young woman.
Mansfield challenges this stereotype by allowing Laura to be concerned, and - more so - by including the final scene. When Laura and Laurie - boy and girl - share a moment of realization about life's priorities, Mansfield shatters both the idea of gender and the idea of class.
The thing you need to know about England and its society is that it is very hierarchial and has been since the invasion of the Normans and the installation of feudalism.
This is very obvious in the story since the party is being held at the Sheridan's home within hearing distance of the poor laborers who live in the nearby cottages. One of the laborers has been killed in a fall, and the family is suffering. Mrs. Sheridan (who seems to be concerned only if there are enough canna lilies and that the man did not die in her garden) gives off an attitude of indifference toward the laborers and is shocked when Laura suggests cancelling the garden party since she feels it might be insensitive to have music and a party when a neighbor is suffering the death of a loved one.
Laura is quickly convinced through hats and party garb that canceling the party isn't necessary. Afterward Laura travels into the "underworld" of the laborers' cottages to give a basket of food to the grieving family. She is quite caught off guard and is consumed by the depressing darkness, the sadness, and the poverty in which they live. Laura comes a little unglued being in this world so foreign to her and begins running in her fright from the cottage world. She runs into her brother who escorts her back to safety and comfort. All of the women seem very weak and incapable in this story--even Laura who tries to do the right thing.