The bored student is Lemonhope, one of Lemongrab's many children / creations; he has been expelled from Lemongrab's kingdom for not being disharmonious, and Princess Bubblegum is trying to explain to him why it's important for him to foil Lemongrab.

Lemongrab is secretly Larry Wilmore. Play this clip and close your eyes:

https://www.youtube.com/watch?v=kdgFfPUOKpw#t=4m12s

Why would Princess Bubblegum try to explain why it's important to foil Lemongrab, by showing Lemonhope a film which *explicitly states* that everything is fine and that Lemonhope should come nowhere near Castle Lemongrab? Would it not be more effective to show Lemonhope a film with message: DO come to Castle Lemongrab?

This show is dumb.

Well the Tart Toter is too incoherent to do so, if that's what you're asking.

Who or what is a Tart Toter?

He totes the tarts, duh.

The whole joke is that PB isn't a very good teacher, Homer. She totally fails to convince Lemonhope to overthrow Lemongrab, and he just wanders off instead. Later he comes back and does stop Lemongrab, not because he cares about the kingdom or his people, but just because folks won't stop telling him it's his duty and he wants them to shut up. After that he wanders off again to find a place where no one will tell him what to do.

It's kind of neat, Lemonhope is basically a character stuck in a Hero's Journey narrative who sees it as a type of prison.

Lemonhope found the Anti-Life Equation and returned to Apokolips to overthrow Lemongrab, but when he got there he refused to use the Equation and beat his father with honor and personal strength.

MmmmYESssss, I think you're right.

Are you implying Kim Jong-un ate his brother?

It's metaphysics: she can create living candy flesh but she cannot imbue her creations with souls. Those that happen to turn out all right do so by coincidence, but even so they are soulless husks.