Welcome back to school, kids!

A 3-foot-long ball python escaped from its cage inside a Wisconsin high school — and is believed to be hiding out inside a science lab ventilation system, school officials told The Post.

The snake — a science teacher’s pet named Kai — is suspected to have busted out of its cage inside a classroom at Fond du Lac High School sometime during the Labor Day holiday weekend, principal Matt Steinbarth said Wednesday.

“As of right now, we’re pretty sure it’s in a ventilation system connected to our science lab,” Steinbarth said. “We could see that it went into one area of it, at least.”

Firefighters in Fond du Lac were unable to track down the nonvenomous constrictor on Tuesday — the first day of classes for freshmen at the 2,000-student school — despite using a thermal imaging camera. The snake’s suspected whereabouts were narrowed down after school officials consulted two herpetologists and a local snake expert, Steinbarth said.

Markings of dust that was displaced by the snake led school officials to the ventilation system. They now believe they’re close to finding the typically docile snake — which tends to curls up in a ball when threatened.

“You could see that it was up there,” Steinbarth said. “You could see it was slithering in the dust.”

It’s the first time the snake has managed to bust out of its cage, the principal said.

“It had been home all summer long and was just recently brought back into the school,” Steinbarth said. “And it popped its lid and found a different place to hide.”

But the principal said the snake, which is often used by a science teacher during biology class, shouldn’t be considered a threat to students.

“When you hear the word ‘python,’ you start to think the worst-case scenario,” Steinbarth said. “But if you read up on ball pythons, it puts you more at ease.”