The averages you calculate are in a column vector called Student_Average, but your subsequent code does not even use that variable. So you need to alter your code to use that variable.
Also, for a beginner, I would advise using a simple for loop to calculate the grades result, which it appears you want to be a char variable.
So an outline of your code would be:
Student_Average = etc.
The code you fill in above is going to be your if-then-else stuff you show above. You will calculate the grade based on Student_Average(k), and you will assign the result to grade(k).
And your current code doesn't assign anything if the average is between 59 and 60. So I would advise changing this line
to this instead
(Side note: What is an 'E' grade?)