How did the Souls give new meaning to the phrase "bottoms up"?
"Bottoms up" is ordinarily a term used to describe taking a drink, as in a toast.
As a result of a recent school reorganization, sixth grade, which used to be the top elementary school grade, is now the bottom grade in middle school (Chapter 3). When the sixth grade Souls defeat the eight grade academic bowl team, they win the right to represent Epiphany Middle School in the district championships. It is unprecendented for a school to be represented by its youngest, bottom class, and Mrs. Sharkey thus says that the Souls give "new meaning to the term 'bottoms up'" (Chapter 6).