In SSMS, go to Help... View Help.

This will pull up Books On Line.

Typing Average in the upper right search box will bring up relevant topics.

If time is of one of the main factors, a pivot table directly in Excel can be added in less than a minute.

Note in your sample data, 2 of the Students have no ID - which is the key to the Student.

Although I am glad you are trying to learn, something just seems odd.

Something as important as grades, and this is such a basic query question.

And a school district managing some of it in Excel?

Do they have any openings? 😀