Preserving a gender-neutral environment is not simple. Carina Sevebjork Saur, 57, who has been teaching at the school for a year and a half, said she often catches herself saying the wrong thing, like an offhand compliment on a child’s appearance.

“You are on your way to saying something, as a reflex, but you have to hold it back,” she said. “You can comment on clothes in other ways: ‘Oh, my goodness, how many polka dots!’”

The effort is compensated, teachers said, by the impression that they are unraveling mysteries, and as spring approached, some changes could be observed among the Penguins.

One of the group’s teachers, Izabell Sandberg, 26, noticed a shift in a 2-year-old girl whose parents dropped her off wearing tights and pale-pink dresses. The girl focused intently on staying clean. If another child took her toys, she would whimper.

“She accepted everything,” Ms. Sandberg said. “And I thought this was very girlie. It was like she was apologizing for taking up space.”

Until, that is, a recent morning, when the girl had put a hat on and carefully arranged bags around herself, preparing to set off on an imaginary expedition. When a classmate tried to walk off with one of her bags, the girl held out the palm of her hand and shouted “No” at such a high volume that Ms. Sandberg’s head swiveled around.

It was something they had been practicing.

By the time March rolled round, the girl had gotten so loud that she drowned out the boys in the class, Ms. Sandberg said. At the end of the day, she was messy. The girl’s parents were less than delighted, she said, and reported that she had become cheeky and defiant at home.

But Ms. Sandberg has plenty of experience explaining the mission to parents.

“This is what we do here, and we are not going to stop it,” she said.