Hillari represents every pretty and mean high school girl from the dawn of time. She is all things popular and superficial. To have a girl who dresses up in prairie dresses sing to her in the lunchroom on a ukulele is the epitome of embarrassment for one like Hillari. Consequently, on the day before Hillari's birthday, she tells Stargirl not to sing to her while jabbing a finger into face at the same time! Very articulately, Stargirl says that she won't sing to Hillari. 

Spinelli describes the lunchroom scene very well by walking the reader through every minute of lunch and what Hillari and Stargirl do through the whole period. Stargirl makes her move at the end of lunch though, and Leo describes it in the following way:

". . . walking, right up to the table where Kevin and I sat with the Hot Seat crew. . . she sang "Happy Birthday." It was Hillari's name at the end of the song, but true to her word of the day before, she did not sing it to Hillari--she sang it to me" (28).

Touche, Stargirl! She kept her word and didn't sing directly to Hillari's face, but she sang to Leo instead. The irony of the situation roasts Hillari and she stomps out of the lunchroom.