Last August (2018) a new version of Classroom was introduced that had new features. These new features included a Classwork page. Teachers did not have to continue using that version and could revert back to the previous version.
As of September 4, 2019 any Google Classroom that does not include Classwork will automatically be converted and the option to return to the previous version will not be available.
If you still have the old version, you can switch to the new version by selecting the ? on the bottom left of the class in question.