Stargirl embarrasses Leo in the lunchroom by coming over in front of everybody and singing "Happy Birthday" to him.
It is Stargirl's idiosyncratic habit to sing to whomever's birthday it is that day in the lunchroom, but even though she sings to Leo, it is actually not his birthday. It is, in fact, Hillari Kimble's birthday, but Hillari has made it quite clear ahead of time that there will be dire consequences if Stargirl performs her routine and sings to her that day. Stargirl still gets up at lunch and sings "Happy Birthday" with Hillari's name as the birthday girl, but, in cheeky deference to Hillari's expressed threat, she does not actually sing the song to Hillari. Instead, she walks right past her and stops at the table where Leo sits with his friends, and sings the song to him.
The students in the lunchroom express their appreciation of Stargirl's handling of the situation "with wild applause." Leo, of course, is confused and completely embarrassed to have been singled out. When Stargirl is asked why she chose Leo to sing to, she adds to his discomfiture by responding mischievously, "He's cute" (Chapter 6).