The main reason that Jess tried to avoid Leslie was because Leslie is a girl. Jess has only sisters and his mother--his father is present, but not very comforting to Jess--and he doesn't want to have another girl in his life. He chooses to ignore her for the most part, but nothing hostile. Then, at school, there is a race in gym class. Jess has been training all summer to be the fastest student in his class--and Leslie easily outruns him. During class at the end of the first week, they sing a song about being free to be yourself. Jess suddenly realizes how cruel he's been to Leslie--and thus their epic friendship begins.