August insists he's not well enough to go trick-or-treating, which Via finds shocking and sad. She thinks that Halloween must be the best for August, since he looks like any other kid. She goes to his room in the evening to check on him and make sure he doesn't want to go to the Halloween parade. He still doesn't want to go, and Via can't figure out how August can be so brave for other serious medical stuff but be sidelined now by some nausea.
In this situation, Via steps into a parental role. Even though she insists she struggles to read August after his surgeries, it's telling that she can still ascertain that something must be wrong other than nausea.
Via asks if there's something more going on, and August, close to tears, finally tells Via what he overheard Jack saying. Via soothingly says that Jack probably didn't mean it, but August won't be comforted. He becomes suddenly angry, punches his pillow, and yells that he hates school. Via lets him cry for a minute and then suggests he put on his Boba Fett costume so they can go to the parade. Finally, he agrees. Via helps him with his straps and belts.
One of the reasons August feels comfortable telling Via what happened is because Via is still a kid, not an adult. This begins to illustrate how part of coming of age means learning to trust the adults in August's life with stuff like this; he tells Via because he's not there yet, though his ability to voice what’s bothering him is a step in the right direction.