Here are some studies and articles regarding the efficaciousness of replacing classroom chairs with stability balls. Please read and review carefully before implementing this technique into your classroom. I want to note that all of these studies involved small sample sizes and the greatest improvement was seen in students with an ADHD or ADD diagnosis; however, there was a generalized improvement in all students. A negative to point out is the tendency for slouching to reemerge after novelty has worn off, which can lead to back pain and/or minor back problems.
Overall, I think this is a positive and effective technique if implemented correctly and introduced gradually. As with any new technique, I would recommend regular follow-up with students to ensure any negative side effects are mitigated before poor habits develop.
Examples of stability balls in and out of the classroom. Links will take you to websites where you can purchase these products.