How can a class teacher access an overview of grades from all subjects in their class?

The administrator can award the class teachers a right to access information about grades from all subjects in their class.
Administrator: How to set up rights in the grade book?

The class teacher can then view the grades via the Advanced mode.
Click on the Grades and open the Select the course view. Advanced mode icon displays.

Select your class and "All subjects" in the Subject field.