Can someone answer my question on Parenthetical Citation in MLA Format?

If I have two sentences that both have the same citation, do I put the citation before the period of the second sentence, or after the period of the second sentence? For example, is this right: Approximately one-half of all clroom time is taken up with activities other than instruction, and bad behavior and disciplining are responsible for a significant portion of this lost instructional time. During most of its 22 year existence, the Annual Gallup Poll of the Public’s Attitudes Toward the Public Schools has identified “lack of discipline” as the most serious problem facing the nation’s educational system (Kotton, "Schoolwide and Clroom Discipline")