Review of linked list and quick sort
First of all I don't know why are you trying to reinvent the wheel? Java has a strong and beautiful API with many methods and there is also a method for sorting an custom array. Which is more faster than quicksort. However I think this is for your own practice.


Your convertA method is not good, cause you are passing the reference as argument as well. So either make the method static or use the existing method toArray().
You are converting a student list to an Node array, why? change Node array to Student array. So this

arr[i].getStudent().getAge()


will change to

arr[i].getAge() 

there is a possible bug you are calculating (left + right) / 2 which may cause an overflow, though Java int range is very big you will face the overflow very rarely, but if you want to fix this; change to (right - left) / 2 + left.


Check How Other Have Done The Quicksort :

From CodeReview 1

From CodeReview 2

Why you should use the API method?