End this nested loop in C.

Start by indenting your code correctly so it doesn't look like it ends with a spurious close curly bracket:

#include <stdio.h> 
#include <limits.h> 
int main () 
{ 
  /* variable definition: */ 
  char StudentName[100];
  char end;
  float ExamValue, Sum, Avg; 
  int students,exams; 
  // Loop through Students 
  for (students=1; students <INT_MAX ; students++)
  { 
    // reset Sum to 0 
    Sum =0.0;   
    printf("Enter Student Name \n"); 
    scanf("%s", StudentName);    
    // Nested Loop for Exams 
    for (exams=0; exams < 3; exams++)
    { 
      printf ("Enter exam grade: \n"); 
      scanf("%f", &ExamValue); 
      Sum += ExamValue;
    }    
    Avg = Sum/3.0; 
    printf( "Average for %s is %f\n",StudentName,Avg);
    if (StudentName == 'end')
    {
      break;
    }
  }
  return 0; 
}

Now what I would look at is moving the "exit test" higher in the function, so that it's the first thing you do:

#include <stdio.h> 
#include <limits.h> 
int main () 
{ 
  /* variable definition: */ 
  char StudentName[100];
  char end;
  float ExamValue, Sum, Avg; 
  int students,exams; 
  // Loop through Students 
  for (students=1; students <INT_MAX ; students++)
  { 
    printf("Enter Student Name \n"); 
    scanf("%s", StudentName);    
    if (strcmp(StudentName, "end") == 0)
    {
      break;
    }
    Sum =0.0;   
    for (exams=0; exams < 3; exams++)
    { 
      printf ("Enter exam grade: \n"); 
      scanf("%f", &ExamValue); 
      Sum += ExamValue;
    }    
    Avg = Sum/3.0; 
    printf( "Average for %s is %f\n",StudentName,Avg);
  }
  return 0; 
}

If that doesn't fix your problem, then you need to explain in a lot more detail exactly what it is doing that you didn't expect, or not doing that you did. And what you have done to find out why - including what the debugger showed you while your code was running.