When none of the students showed up to cheer for her after her oratorical win, what did Susan/Stargirl do the following Monday?

She returns to her old self, Susan, and begins sharing cookies to her school mates which they refuse to eat.

Explanation:

'Stargirl', by Jerry Spinelli, tells the story of a young girl named Susan who had been home-schooled before being transferred to Mica High School. While there the students found it hard to accept her because her style and sense of dressing was odd to them. Susan however was a star-girl who excelled in her academics. Part of the anger of her mates was that she was kind and rejoiced when everyone was winning.

An oratorical competition was organized by her school in which she won. She was only cheered by Leo and Doris, her two faithful friends as well as some teachers. Other students ignored her. So, when she came back to school on Monday, she quit trying to impress the students and returned to her abnormal dressing. She also started offering cookies to her school mates passing by. The cookies were rejected by the students.