Teachers once entered and totaled their pupils' grades in actual grade books, but these days the "grade book" is probably a spreadsheet file. Gradebook for Excel from Vertex42 is a free pack of grade-keeping spreadsheets for Microsoft Excel. It's aimed at college and high school educators, but it's suitable for any teacher. It includes templates for grading based on percentages, points, and grade point averages (GPAs) or letter grades.
This is one of the developer's vast number of offerings of templates for Word, Excel, and other programs, many of them freely available. It's for teachers, professors, substitutes, coaches, or anyone with Excel who can use a good grade book.