GREEN — Wiggling is rampant in Carol Sherwood’s fourth- and fifth-grade classroom. Fourth-grader Ephraim Mullins, 9, bobs on a big red yoga ball during silent reading. “I’m a big bouncer,” he said with pride. “It’s like bouncing on a trampoline.” The students sit on large, inflatable yoga balls instead of traditional chairs. Sherwood, a teacher at Sunnyslope Elementary, introduced the exercise balls to her classroom four years ago after learning that sitting on one helps with balance and core strength. The ball forces muscle engagement and increases blood flow, leading to more alertness. The alternative to chairs was first tried …