Who did Leslie hear crying in the girls bathroom

Leslie hears Janice Avery crying in the bathroom. Janice Avery is a seventh grader who is described earlier in the book as being "the one person who devoted her entire life to scaring the wits out of anyone smaller than she."
Leslie tells Jess that she heard someone crying and is sure it was Janice Avery. Katherine Paterson writes:

Leslie came out at recess to tell Jess that she had started into the girls' room only to be stopped by the sound of crying from one of the stalls. She lowered her voice. "This sounds crazy," she said. "But from the feet, I'm sure it's Janice Avery in there."

Leslie goes on to say that the shoes on the feet in the bathroom have "Willard Hughes" written on them and crossed out. Janice is the only girl with such shoes.
Jess encourages Leslie to go back into the bathroom to help Janice, explaining, "if she was an animal predator, we'd be obliged to help her." Leslie is hesitant but eventually agrees.
When she speaks to Jess later, Leslie explains that Janice told two of her female friends about the way her father beats her. According to Leslie, it is the kind of beating that causes people to go to prison. Bobby Sue and Wilma, Janice's friends, told everyone about her father beating her. That is why Janice was crying in the bathroom. Leslie tells Janice to pretend she does not know what the girls are talking about.