Of course, by Monday Jess knew; but still, but still, at the bus stop he looked up, half expecting to see her running up across the field, her lovely, even, rhythmic run. (13.44)
Jess may only be in the fifth grade, but he'd probably have this reaction to Leslie's death no matter how old he was. It's going to take him a while to realize, to remember, that Leslie won't "run up" to the bus anymore, that he won't "see her running" again at all. Just because he knows she won't be there, doesn't mean he's not going to expect or desire it. She's his best friend. He doesn't want her to be gone.