On a positive side, you know
Java syntax as you wrote quite a lot code already. The problem it seems you're facing currently is - lack of testing before you move onto another task.
Keep this code as a reference, some of the parts you'll be able to to re-use probably. Some of them are just wrong, so no harm to delete.
I hope you're clear with your requirements of program and have read them three times at least to make sure you understand them well.
If you haven't:
1. Read requirements once again, only the very first task (omit the rest, these not needed now as it is not a complex system)
2. Create new .java file
3. Define class name, not Proj5 - that could be in the general comment of the class if it needs to be mentioned somewhere. Consider naming it GradeSystem or similar.
4. Create main method and populate with 1 line of code. i.e.:
5. Create method which takes user input, NOW you use some of your code from your previously written code, see code below, make it a method:
and it would look like
later you'll be able to call that method and test it if it works as expected:
Quickest way to test it, is to run that program, input number 4 and print that out, with:
6. So, ONLY when you're sure about the correctness of the present code, you move forward. For your reference what the code looks like at that time:
And you do all the same with the rest of your program. Write 1 method (3 lines of code - compile - 3 lines of code - compile), test it. Move onto the next task.