Can you assist to improve this class please?

First thing is programming is always to make a good sketch of your system, for which you're going to write the application for, or atleast the logic for.

In your case, which seems more like an institute's system to save the student's overall performance. In that case, you should move importantly focus on the student himself and these other classes must not be of string data type but of their own class's data type. Such as this one,

class Performance
{
    public Academic Academic {get; set;}
    public Physical Physical {get; set;}
    public Socials Socials {get; set;}
}

.. now to make them work, you must have corresponding classes for them to control the data being passed, like this maybe,

class Academic {
   public string static Course {get; set}
   public string static Semester {get; set;}
   // other properties here
}

You can build other two classes, (Physical and Socials) like above one that would hold their own data in them. In this way, you can define the classes and objects to contain the related data in them. Now, you would know that when you're going to call the properties, you would be able to call them like,

Performance performance = new Performance();
performance.Academic.Course; // because Course is a static field

But it won't let you access the Semester field, because there will be an error saying, "string" doesn't contain any definition of "Semester". That is legit, because Semester is not a property or member of System.String[^] or string.

I really would like to recommend that you first of all, start to learn how to use the C# language, and what are objects and how to call these properties from the code. Then you will be able to understand these features and might be able to create a long query by each member returning an object that would allow the next member to be called and so on and so forth.

One more thing, I did notice that there was an ambiguity in your identifiers for members being called and inside the class. You should not use any typos while writing the names, C# is a case-sensitive and letter-sensitive language. Names of the identifiers and members must match in order to compile as it is, the source code won't even compile.

Good luck!