Covalent and Ionic Bonds
Bonds are usually used to describe how electrons are shared. "Partially ionic bonds" are polar covalent bonds. Which is not the same as saying that a compound has different types of bonds in the whole compound. I think that your teacher gave you a bad question and a bad answer. If the question had asked whether all the bonds in sodium nitrate are ionic, then it would be a different story.

However, it is true that the bonds between the nitrogen and oxygen are covalent. Nevertheless, stating that sodium nitrate is ionic shouldn't be wrong.