So Close!
The now mostly unsupported iStudiez Pro app has a good implementation of this feature. I think if something like that were to be added to this app, then it would be more worth it to use than my school-provided Outlook Calendar and Microsoft To-Do. I don’t see any other apps out there really doing this, either, and the aforementioned iStudiez has lost developer support, so there is a niche.
I don’t believe it would be too hard to add some fields to input grades for each assignment/exam, enter the weight distribution for each course, calculate the GPA based on a user-inputted scale, display real-time course grade and semester GPA, etc. Would definitely use the app then.