From Jerry Spinelli's Stargirl, why did Hillari tell Stargirl not to sing to her on her birthday? How did Stargirl keep her promise?

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 tellsnot 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. Theof the situation roasts Hillari and
she stomps out of the lunchroom.