HOW HAS Netflix’s otherwise promising new “Magic School Bus” reboot so badly whiffed on a single crucial step?

The trailer for “The Magic School Bus Rides Again” has arrived in advance of its Sept. 29 debut on Netflix, and there are reasons to be optimistic. Lily Tomlin returns to her Daytime Emmy-winning role as Ms. Frizzle (now elevated to Professor Frizzle). Lin-Manuel Miranda brings amiable warmth to the theme song, dialing down the vocal crackle of Little Richard’s original track. And Kate McKinnon — so good as a voice performer in the current film “Leap!” — inherits the bus-driving role as Ms. Frizzle’s kid sister, Fiona Felicity.

With so much authentic talent involved, then, it’s a pity that the show’s guiding forces decided to alter one key component of the original: visual soul.

The ’90s cartoon series was a Canadian-American production from Scholastic, based on the hit book series, that sought to make a STEM education more entertaining. And as one of PBS’s first animated shows, “The Magic School Bus” boasted a warm, expressive visual style that felt reassuringly handcrafted. Beyond the extreme squash and stretch of that wild bus, the affecting core of the show was how the engaging dialogue emerged from faces we could relate to — with a retro aesthetic that nodded to such classics as “Charlotte’s Web.”

Two decades later, however, that warmth has been painted over like an urban mural lost to a gleaming new condo development.

The reboot has saddled its talent with the plastic facial expressions of a children’s Barbie or Bratz ad. It’s as if the artists have embalmed the soul of the show beneath the most plastic of pixels.

“The Magic School Bus Rides Again.” (Netflix/Scholastic)

Now, this shift may not faze a new generation of “School Bus” riders. But for fans of the original series, it’s almost a large swinging arm of a bus signal that reads: “You’re no longer welcome here — move along.”

Let’s hope the show might save the slickest of CGI effects for the settings of the new adventures, and return some warmth to the cutout characters.

Beyond that, at least we’ll always have Lily, however gilded she may now be.

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