In the book Wonder, there is a ceremony at school. What happens at the ceremony that shows a change from the beginning to the end of the school year?

The answer to your question can be found in chapter 121 of Wonder. What happens at the award ceremony and graduation is that the student body gives August a standing ovation for winning the Henry Ward Beecher medal: a medal given to a “notable or exemplary” student in the class. This is different from the beginning of the year when no one except for Jack would sit next to August in homeroom.

As August walks up to the stage to receive his award, August feels like he is "floating," which is where the title of the chapter comes from. This is another difference. At the beginning of the school year, August thought no one would want to acknowledge his presence, let alone give him an award and honor him in front of the entire school. Now August realizes his self-worth. August realizes that he is called brave and extraordinary because he faces incredibly difficult challenges.