Not many classrooms have large rocks as seats, soft earth instead of tile floors and dense trees as sheltering walls. But the Elachee Nature Academy has offered such an outdoor learning space for preschoolers for years, and this fall, the program will include a kindergarten and first-grade classroom as well. It’s the only accredited, nature-based school in the region, allowing children 50 percent of their school day in the classroom and the other 50 percent in lessons outdoors at Elachee Nature Science Center.