When Calvin brings up that example of the other kid in class regarding the "best day", Chance doesn't recognize the name at first, Calvin mentions some other detail and Chance groans and notes that the boy pulls his pants to his ankles when he pees, but Calvin continues his point: A few weeks ago, the class had a pizza party, that was on the kid's birthday, and the kid said it was the best day of his life. And Calvin thought to himself, at that kid's age, yeah, maybe it really was the best day of his life so far. Then, morbidly, he turns to Chance and asks what if the best day of his life has already passed? (We need to work on exactly what Calvin says, because he's subtly implying he's going to kill himself soon.)
Chance gets upset at this, saying Calvin is being childish. Calvin responds but they are children. Chance says no, they're not, not like the other. This is the segue into Calvin asking if he ever wants to be like them, nodding at the kids on the playground. Chance says no, making his rude observation regarding what counts as a highlight in the day of a 'normal child'.
The bell rings, but Chance knows it's the end of the school year, he won't see Calvin again, and he implores Calvin to tell him more about Hobbes ("what I saw before") saying he knows what that thing is, he's seen one before and it's more dangerous than Calvin realizes, Calvin won't be able to control it. Calvin, being shitty, merely responds to Chance that he "should get some sleep" referencing the nightmares Chance mentions earlier in the scene.
It's Chance pulling Calvin's arm that makes Calvin annoyed and when he turns back he's transitioned to the next scene.
Also when he sits down during the transition with the birthday cake he looks at it strangely and says his birthday isn't until tomorrow. Angela looks concerned and says as she told him, they won't be here tomorrow. He nods, as if suddenly remembering as the transition wears off.