Is your grade book a dedicated app (desktop? mobile?) or just some kind of online app in a web browser? I presume there's no import feature anywhere, is there? If you could find any way to import grades, it might be better to find a way to transform the shape of the other exported spreadsheet into some other format the grade book will import.
Unfortunately, all of my other ideas would require a fair bit of technical programming skill, and they would both assume your grade book tool is in a web app.
For automating web stuff, Selenium WebDriver
is a popular tool for programmers and SDET/QA engineers. You write code that teaches it what to look for in a web page and how to interact with it (click, type, etc). At the lower level, most languages like Python have libraries for communicating directly over HTTP, parsing web pages, and posting form contents; requests
and Beautiful Soup
come to mind. If you're not already a web developer, though, both of these answers are probably way
out of scope for you.
Outside of web apps, the tooling for automating clicks and types gets real shaky. With macOS, technically
it's possible to use AppleScript to automate UI interactions, but it's a real nightmare finding the appropriate objects and the APIs are super obtuse and nigh-impossible to find good documentation. I know this because I have written two AppleScripts that change a bunch of system settings at night and in the morning, and it was agonizing
getting them to work as I wanted.
I've also seen/touched a program that did UI automation for Windows apps many years ago, so I know that's possible too, but I can't remember the name and wouldn't be able to give any good recommendations.