Is interest earned on us savings bonds exempt if used to open a 529 plan for my granddaughter who is still in High School? If so, how do I enter it on the tax return?

Savings bonds were purchased in my name thru a payroll deduction plan offered by my employer starting in September 1983.  I cashed them in 2017 and used all proceeds to open a 529 plan for my granddaughter.  I was told that the interest earned on these bonds is tax exempt if used to open a 529 plan.  How do I enter this information?  Thank you!