Imagine a day filled with nature walks, playing in the mud, studying worms up close and reading stories in the forest. Doesn't really sound like school, does it?

Welcome to the world of nature preschool — where young learners spend part or all of their school day outdoors.

There are more than 150 nature preschools across the country, up from about two dozen in 2012, said Christy Merrick, director of the Natural Start Alliance, a national coalition that promotes outdoor education in early childhood.

Merrick attributes that growth to recent research showing the importance of high-quality early childhood education and the importance of nature.

Richard Louv discusses this impact in his books "Last Child in the Woods" and "The Nature Principle." He contends social and technological changes in the last few decades have led to less time in the natural world for many of us — what he calls "nature deficit disorder."

"Technology now dominates almost every aspect of our lives," Louv said. "Technology is not, in itself, the enemy, but our lack of balance is lethal."

Those concerns — coupled with a heavy emphasis in standardized testing in recent years — have led some parents to consider a more experiential, nature-centered education for their kids, Merrick said.

"I think a lot of people are really concerned about how children have so much screen time," she said. "These schools offer something different; literally a breath of fresh air for parents."

At the Fiddleheads Forest School, nature is the classroom.

A typical day at the Seattle-based nature preschool might consist of singing songs, reading stories or taking a walk to the pond — all outdoors, within the University of Washington Botanic Gardens. The school was founded in 2013 by directors Kit Harrington and Sarah Heller.

"The most wonderful thing about the outdoor classroom is everything is a material for learning," Harrington said. "I think that is something you’re not going to find in an indoor environment."

While each day is structured, kids also get free time to play and explore.

"Kids develop a persistence and a grit," Harrington said. "They learn it’s OK to be outdoors in any sort of weather; they can challenge themselves to go further and learn new things."

The school currently serves 50 families, but has more demand than it’s been able to meet, Heller said. "There’s a lot of parents who grew up spending an enormous amount of time outside and they want that experience for their kids," she said.

Stacy McReynolds, vice president of education at the San Antonio Zoo, says she often hears parents say they wish an outdoor preschool had existed when they were young. McReynolds operates the San Antonio Zoo’s Zoo School, a nature preschool where children ages 3-5 spend most of the day outside learning and exploring.

About 49 kids are enrolled at the school, founded in 2004, which all takes place on the zoo’s grounds. Class happens every day, rain or shine.

"We are very child-centered and focus on what sparks kids’ creativity," McReynolds said. "If you’re going to be counting, why not count the number of monkeys who came to get the treat you made them?"

Rather than using flash cards or worksheets to learn math and science, students might spend time weighing materials in the mud kitchen or counting the number of green snakes they find. Lessons all have a real-world application, are open-ended and are guided by kids’ curiosity — so sometimes the focus may shift depending on what the class is interested in.

"If they get fascinated by web worms, then that’s what we’re going to do," she said.

About 140 families are on the school’s waiting list, but the preschool will be able to serve up to 200 families after its planned expansion.

The trend of incorporating nature into education is likely to continue growing as educators and parents learn more about the relationship between nature experience and healthy child development, Louv said. But he says it’s going to take a group effort.

"Teachers and schools can't go it alone," Louv said. "Parents and whole communities must pitch in."