Robbi Giuliano teaches her fifth grade class as they sit on yoga balls at Westtown-Thornbury Elementary School Monday, Feb. 4, 2013, in West Chester, Pa. Replacing stationary seats with inflatable bouncers has raised productivity in her fifth-graders at Westtown-Thornbury Elementary School, making students better able to focus on lessons while improving their balance and core strength, she said. (AP Photo/Matt Rourke)

Robbi Giuliano teaches her fifth grade class as they sit on yoga balls at Westtown-Thornbury Elementary School Monday, Feb. 4, 2013, in West Chester, Pa. By making the sitter work to stay balanced, the balls force muscle engagement and increased blood flow, leading to more alertness. (AP Photo/Matt Rourke)

Students in Robbi Giuliano's fifth grade class sit on yoga balls as they complete their assignments at Westtown-Thornbury Elementary School Monday, Feb. 4, 2013, in West Chester, Pa. By making the sitter work to stay balanced, the balls force muscle engagement and increased blood flow, leading to more alertness. (AP Photo/Matt Rourke)

Students in Robbi Giuliano's fifth grade class sit on yoga balls as they complete their assignments at Westtown-Thornbury Elementary School Monday, Feb. 4, 2013, in West Chester, Pa. The exercise gear is part a larger effort to modernize schools based on research linking physical activity with better learning, said John Kilbourne, a professor of movement science at Grand View State University in Allendale, Mich. (AP Photo/Matt Rourke)

Robbi Giuliano teaches her fifth grade class as they sit on yoga balls at Westtown-Thornbury Elementary School Monday, Feb. 4, 2013, in West Chester, Pa. Giuliano says "I'm able to get a lot done with them because they're sitting on yoga balls." (AP Photo/Matt Rourke)

Robbi Giuliano teaches her fifth grade class as they sit on yoga balls at Westtown-Thornbury Elementary School Monday, Feb. 4, 2013, in West Chester, Pa. Replacing stationary seats with inflatable bouncers has raised productivity in her fifth-graders at Westtown-Thornbury Elementary School. (AP Photo/Matt Rourke)

Students in Robbi Giuliano's fifth grade class sit on yoga balls as they complete their assignments at Westtown-Thornbury Elementary School Monday, Feb. 4, 2013, in West Chester, Pa. By making the sitter work to stay balanced, the balls force muscle engagement and increased blood flow, leading to more alertness. (AP Photo/Matt Rourke)