We need to be very careful when examining the character of Laura and what she might represent in this excellent short story. Certainly, you are right that, at the beginning of the story at least, she seems to represent a new understanding of the social order with her disavowel of class distinctions. Note her thoughts about her solidarity with the men who come to put up the marquee:
It was all the fault, she decided... of these absurd class distinctions. Well, for her part, she didn't feel them. Not a bit, not an atom...
However, in spite of such spirited declarations, the way that her mother is able to tempt her into forgetting about Mr. Scott and his unfortunate demise through a new hat and her participation in and enjoyment of the party suggests that she comes to act on these "absurd class distinctions" just as much as her sister and mother.
However, the one ambiguous note comes at the end, when Laura contemplates the body of Mr. Scott, and sees that he is at peace:
What did garden-parties and baskets and lace frocks matter to him? He was far from all those things. He was wonderful, beautiful. While they were laughing and while the band was playing, this marvel had come to the lane.
Such lines suggest that Laura comes to reflect on her own life and existence in the upper class of which she is a part and judges it, finding something beautiful and eternal in the silence of the dead man. This could be used as evidence to suggest that Laura does indeed represent a new understanding of society after the horrors of WWI.