Classes in SyncSketch are called "Projects". It is recommended to create a project for each class e.g "Animation Winter 2018". Projects contain reviews which are equivalent to an assignment into which the students can upload their work.
To create a new class you can hover over "Projects" in the top navigation and click on "Add Project" to open the following dialog:
Please make sure to select the right account when creating your project.
For more information on creating projects you can also head over to the following article: Adding and Deleting Projects