How to add a teacher the right to view grades in multiple classes?

The rights to view and edit grades is based on the courses created. When the teacher has created a "course", he can edit grades in it. It is no necessary for the administrator to set this right for the teacher elsewhere.

The administrator can set class teachers the right to view grades from all subjects of their class.
link: e1418

If you want to add the right to view grades for, proceed as follows:

Log in as an administrator. In the left menu, click Overview and then select the Rights: Grades

Mark entire classes by clicking on the class name in the left column. If you hold down the CTRL key, you can mark several classes at once.
On the right side of the screen, scroll to the bottom where you can find advanced rights for editing and viewing grades. Add a teacher in the View Grades section.

More help: Grade book setup