And in almost all cases it's better to use vector instead of array:
Code:
#include <iostream>
#include <string>
#include <vector>

using namespace std;

int main () 
{

     vector<string> ftr(15);

     cout << "Enter the grades of the student one by one" << endl;

     for (int i=0; i < ftr.size(); i++)
     {
          cout << "Enter the Student Number" << endl;
          getline(cout, ftr[i]);
     }

     return 0;
}