As Justin and Via are kissing goodbye, Mom, August, and Jack come down the stairs. Mom pretends not to notice, but August and Jack giggle. Mom asks Justin to walk Jack to the bus stop and wait with him. As Justin and Jack walk, Jack insists he doesn't need supervision, but Justin decides to wait with him anyway. He gives Jack money to buy some gum and watches him cross the street to the store. Justin thinks that Jack seems too small to be out by himself, and thinks that someday, he'll be an overprotective dad so his kids know he cares.
When Justin uses what he sees in front of him to reflect on the parent he wants to be in the future, it shows that individuals do have the power to break out of the roles that others modeled for them as children (which offers some hope for Julian's fate). However, this also recalls Via’s earlier assertion that she doesn't want to have children.
After a minute, Justin watches three kids, two of whom look much older, tease Jack as Jack walks out of the store. Jack seems upset when he gets back to the bus stop and explains that the kids were Julian, Henry, and Miles. He says they don't tease him during school but explains that there's a war going on. He hands Justin a piece of paper with three lists of names on it and says that Julian turned the whole grade against him.
Jack's willingness to confide in Justin suggests that he does wish he had more support. Like August confiding in Via, Jack is essentially asking Justin for help here, which suggests he doesn't trust the teachers and adults with real power to do much for him at school.
After a few minutes, Justin realizes that the war is going on because Jack is friends with August. Justin tells Jack that middle school is awful, but it gets better. He waves Jack onto the bus and then heads for the subway, where he sees the three boys laughing near the entrance. Angry, Justin tucks his fiddle case under his arm, looks mean, and approaches the boys. He tells them not to mess with Jack and taps his case for effect.
It's important to recognize that Justin is doing the work here that parents and teachers either can't or won't do by telling Julian, Henry, and Miles to knock it off, showing again that adults or even older kids have a responsibility to guide the younger kids towards good, kind behavior.