Students can be added to a Class if they have been enrolled into the Course that this Class belongs to.
To add a student go to the Classes page by clicking on the Classes icon in the main menu. At the top right of each Class container you will see 3 dots (see image below), which when clicked brings up an 'Add Student' link.
Clicking on the link will bring up the names of all students that have been enrolled in this course but have not been allotted to any particular class (see image below).
You can now choose from the listed students and click on Create. This will add the selected students to this class.
The option to add students to a class is also available as a Add New link in the students list page for the Class.