If there will be no tax rate increase, why does the ballot include this statement: "levying of the tax in payment thereof"?

The bond proposition does authorize the district to levy a tax to pay for the new bonds. Bond holders wouldn't purchase the bonds without this pledge. However, as explained in the question above, the school district has paid off a good portion of its current debt that was incurred as the district was growing rapidly. As a result, the payments on current debt will be declining significantly over the next five years. The financing plan is to sell these new bonds over the next five years in a manner that does not increase the tax rate. View Plano ISD's Proposed 2016 Bond Program Tax Rate Analysis