I would want to start by looking at the way that story begins before the ball actually starts, with Leila's trip to the ball in the cab with the Sheridan girls. What this does is that it allows the author to both focus on Leila's feelings of excitement about the ball but also to raise the tension for both us and her as we are left to wonder what the ball will actually be like and whether it will meet or fall short of her expectations. This section of the story gives us very important information about Leila's character, and presents her as a very excited and young girl. Note the following quote when she comparaes herself to the Sheridan girls:
Oh dear, how hard it was to be indifferent like the others! She tried not to smile too much; she tried not to care. But every single thing was so new and excited... She would remember for ever.
The theme of excitement is therefore presented to us through Leila's feelings as she awaits the ball with so much anticipation and also structurally through Mansfield's choice of beginning the story before the ball starts, so that we can see just how excited Leila is.