You need to assign an instance of your lists to each of the lists you are trying to use, as in:
private List<int> _student_rollnumber = new List<int>();
If you don't then when you try to use them you will get a null reference exception.
Additionally, don't use
public
on internal fields - it exposes your code internals to the outside world, and fixes you to use that for the rest of time. Instead, provide public properties and methods to manipulate internal data - that way you can change how you store information without changing the working code that uses your class.
By the way, I would have a class Student which had a roll number, and a name, then then set up a list of Students - that way the Student can take care of itself, it doesn't need to know other students even exist, much less how they are stored.
[edit]HTML characters fixed greater than disappearance.![/edit]