Thinking about Artopia in its capacity as an educational program, the first series is about laying down the groundwork - sharing, listening, being polite, and so on. We meet our four protagonists- Arin, Ezra, Pepper and Hylie- as well as our antagonists- Blanq, Scrubb, and Turp & Tine- and supporting cast- Pencelope, the Elders (who also serve as narrators, when need be) and Pascel. We also get our setting intro, showing off Artopia and Monochromopolis, and a few brief areas of the Shadelands and the wider Kingdom of Artopia. (Supplementary note, they don't have a king, it's moreso just because they thought it sounded better than the Republic of Artopia)
Season 2 introduces more teaching characters as we begin to get into lessons. This season also introduces Woolbur, but mostly as a comic relief character (with B plots revolving around him trying to find something he likes doing). In no particular order:
Inknatius covers history, though his stern demeanour often leads to the kids bunking off (note 2, Artopian history is probably different from ours, so he spends a lot of time talking about how to study things instead).
Paintricia's art lessons go down much better, due to her excitable nature and high energy. The kids still leave, because of course they do, but it's more like an impromptu field trip
Al Crylic starts teaching physics (which is probably the only science that makes logical sense in an art world) and is wacky enough the kids enjoy his lessons. More impromptu field trips, but he also occasionally knocks himself out anvil-on-head style and they just walk out
Pencelope does language, probably English. Ideally, her segments would be done in such a way that they can be translated into other languages and act as EFL stuff. The kids already know they can cause trouble here
Geoff and Dimitri, the geometry twins are in charge of mathematics. I'd like to imagine they aren't always down for chaos, but when they are the kids absolutely know about it
This is also where we see Blanq starting to reach out, talking to Pencelope about maybe setting up traps that require they use the things they learned in the lessons to escape. Nothing life threatening, and they can all be sprung from the outside, but still
Season 3 introduces more new characters: Stacy O'Neary, who runs a craft supplies store; India Tag, who manages the city's treasury (she doesn't show up often here, she'll be more prominent in Adventures Beyond Artopia); Howell Punch, who is imagine is always worried about something (he needs filling out a bit); and Robbin the Bobbin, who I think is one of Woolbur's in-laws. I'd imagine we have a double-length holiday episode, featuring a range of holidays (because it feels silly for a show about teaching kids to be curious and creative to only do Christmas). We also see Woolbur start trying out more things, occasionally getting other characters involved
Season 4 is probably where I'd call it done- the kids are starting to connect with people around them, bonding together and reconciling with Blanq; Scrubb has a bit of an existential crisis over having to stand down before getting consoled by Woolbur, who teaches him that not knowing your purpose is okay, there's more to life than chasing that horizon; Blanq and Pencelope start talking and hit it off, leading to their decision to explore more of the world; Woolbur finds an old abandoned train in a shed and discovers his true passion; and Turp & Time probably end up making cookies for everyone. Seems like something they'd do. And the final episode helps tie it all up in a bow, complete with photo montage credits:
The protagonists, teachers and Blanq celebrating their graduation
Blanq and Pencelope looking at ancient maps
Woolbur teaching Scrubb how to work with the engine (immediately followed up by Scrubb running away scared as it whistles to hide in the ticket office, then discovering the joys of being stationmaster)
Ezra and Hylie starting a mystery club
Arin and Robbin riding the train with Woolbur
Al accidentally blowing himself up in front of Pascel
Paintricia and Inknatius doing joint lessons together
And a photo of the protagonists just hanging out at the top of a hill, looking out over the sunset, as we fade into the photo for the closing shot
And of course, after the closer, we pan upward to look at the night sky, and we get a "Thanks for watching!" Painted across the stars.