First of all...Welcome to TechIMO!!
Here's a few conceptual thoughts to get you started despite our lax little guideline...
--Adding a grade for a student is basically taking a value (cin) and setting it equal to that student's spot in the array (a[0]=x) (each student seems to be represented by and array index 0-5, or six students total).
--Updating is the same process of code.
--Displaying a grade can be a simple cout statement (student [index] has [value] in the class). While displaying the values for all students is the same thing but in some sort of for/while loop running through a number of times equal to the number of students.
--Sorting...there are hundreds of easy ways to do this. The simplest was would be to do some sort of a bubble sort on your array of student grades (look up bubble sort on google to get some code or pseudocode for the algorithm...it's not difficult to figure out).
BTW, goto is not a command recognized in C++, but rather in BASIC and FORTRAN (i think...)
~edit: LOL Redwolf that's not friendly