It is midday at Khepera Charter School in Mount Airy, and about two dozen middle schoolers are standing on the grass, staring at trees. Their science teacher, Kim Johnson, offers clues as they try to identify Japanese maple, sugar maple, pine oak, and spruce.

Such excursions are common at the school, where the lawns and a small vegetable garden have been an "outdoor classroom" for five years. English teachers read poems and have kids write essays in the sunshine. In math class, students measure the lawn's perimeter or solve problems while watching squirrels play.

The point, said Johnson, whose students call her Mama Omatayo or Mama O. (to foster a sense of family, faculty are referred to as Mama or Baba), is to provide learning experiences outside the four walls and to connect children with nature.

Rita Stevens, a special-education teacher at Philadelphia's Huey Elementary School at 52d and Pine Streets, takes a similar tack. She has been using the school's vegetable and flower garden to motivate her third to fifth graders to learn to read and write. Between weeding and watering, the kids label both the plants ("tomatoes" and "peppers") and their parts ("stem," "leaves" and "fruit").

"The kids are hands-on and they will learn how to read words associated with something," explained Stevens. "They'll make the effort to learn about what they're working with."

Learning experiences like these are why Johnson and Stevens joined 48 fellow educators at the third annual Green City Teachers workshop in July, sponsored by the Pennsylvania Horticultural Society. The two-day event trains educators in safe and best planting practices through hands-on work in one school's garden.

This year, Luis Muñoz-Marin Elementary School at Third and Ontario Streets in North Philadelphia was the host. On patches of soil behind the school and in the parking lot, teachers weeded, swept, built wooden frames for seed beds, tested soil for lead, applied mulch, and planted trees and vegetables. They also bonded over shared goals, challenges, and a desire to spread the word: Whether indoors or outdoors, large or small, gardens are a valuable educational tool.

Part of that value is rooted in health issues. In Philadelphia and Southeast Pennsylvania, more than 30 percent of kindergarten to sixth graders were at risk of becoming overweight or were already obese during the 2006-07 school year, according to data from the Public Health Management Corp.

In Philadelphia, and Burlington and Gloucester Counties in New Jersey, school districts have partnered with groups such as the Urban Nutrition Initiative and Kindergarten Initiatives to expand on guidelines of the USDA's Supplemental Nutrition Assistance Program, or SNAP-Ed. In Camden, where access to fresh food is difficult, residents are being helped by the Robert Wood Johnson Foundation's Childhood Obesity Prevention Initiative.

To change habits, though, the message needs to go directly to classrooms. This year's crop of workshop participants, who hailed from schools in South, West and North Philadelphia, Fairmount, Germantown, and parts of Montgomery County, represented educators, schools and nonprofits collaborating to show children how much fun growing food can be.

"It's not an aside. It's part of life," said Al Gaspar, a science teacher at Lansdowne Friends School. "And [they] see that it's not just putting stuff in the ground. It's mulch and composting and watering and harvesting."

Marin Elementary had planned to do something simple, said Principal Abigail Jimenez-Padron. But once the horticultural society and the Food Trust got involved, the project just kept growing.

Candida Rosario, the school nurse, added that the garden would build from the school's existing wellness program, which focuses on physical activity and healthful eating.

"There's such a problem with obesity," she said. "[We] want the children to learn that healthy food is good for them and they don't have to go to the corner store."

For Jessica Moreno, 13, and Desiree Santa, 14, the opportunity to work outside the classroom is a welcome change from their daily routines. The two honor-roll students spoke to the visiting teacher volunteers, explaining the impact that the garden is already having on them.

"In my other school, we had plants and saw how they grow, but nothing like this," exclaimed Jessica, who transferred to Marin last year and will enter the eighth grade. "It's something new. And if you do it, people see what you're doing and will come help."

Desiree, who just graduated and will be attending Franklin Learning Center in the fall, was reminded of the responsibility it places on older students.

"The little kids look to us as role models," she said. "So we want to set a good example. This is something where we can make an effort and really make a difference."

Ensuring that every student gets the most out of the project will require parent and community interaction as well, say Marin's nutrition educators, Adam Utley and Carmen Adler Sanchez, who led the gardening initiative.

After the 50 visiting educators listened to the students speak, they continued to pull at weeds and hammer nails with gusto. There's no one way to get it right, though, said Gaspar.

"No one's a know-it-all," he said. "Every garden space is different. ... Every step, you learn more."

Marin's administrators want their students to learn about botany, the math involved in planning and planting a successful garden, how to spell herb names, and what types of plants are edible. They also want them to become teachers themselves - to help their parents identify healthier foods and start a garden at home to save on food costs and share their bounty with neighbors.

"I think it's going to be a whole sense of community," said Sanchez, "and will also give kids a sense of pride, a sense of belonging."