Сортировка по полю структуры

#include <iostream> 
#include <string.h> 
using namespace std; 
struct STUDENT 
{ 
char surname[12]; 
int group;
int history,math,opam; 
float avrmark;  
}; 
void StudentInput(STUDENT s[], int n);
void FindAverage(STUDENT s[], int n); 
void Sort(STUDENT s[], int n);
int main() 
{ 
STUDENT s[100]; 
int i,j,a,m;    
cin>>a;  
cin.ignore();  
StudentInput(s, a);  
FindAverage(s,a);
Sort(s,a);  
} 
void StudentInput(STUDENT s[], int n) 
{ 
for (int i=0;i<n;i++)  
{  
cout<<"\nStudents surnamename "<<i+1<<": ";  
cin>>s[i].surname;  
cin.ignore();  
cout<<"\nStudents numer group "<<i+1<<": ";  
cin>>s[i].group;
cout<<"Student mark of history "<<": ";  
cin>>s[i].history;  
cout<<"Student mark of math "<<": "; 
cin>>s[i].math; 
cout<<"Student mark of opam "<<": "; 
cin>>s[i].opam; 
cin.ignore();  
} }  
void FindAverage(STUDENT s[], int n) 
{
  int sm, sh, so;
  for (int i=0;i<n;i++)
  {
      s[i].avrmark = (float(s[i].math + s[i].history + s[i].opam))/3;
      sm += s[i].math; 
      sh += s[i].history; 
      so += s[i].opam;
    }
  }
void Sort(STUDENT *s, int n) 
{ 
int temp; 
for (int i = 0; i < n - 1; i++) 
{ 
if (s[i].avrmark > s[i + 1].avrmark) 
{ 
temp = s[i].group; 
s[i].group = s[i + 1].group; 
s[i + 1].group = temp; 
} } 
cout << "Groups for lowering average grade marks"<<"-----------------------------------------------\n";  
for (int i=0;i<n;i++)
{
    cout<<"Group number : "<<s[i].group<<endl;
    cout << s[i].surname << "\t" <<"Average mark  "<<s[i].avrmark <<"\n";
    cout<<"Marks :"<<endl;
    cout<< "History "<<s[i].history<<endl;
    cout<< "Math "<<s[i].math<<endl;
    cout<< "Opam "<<s[i].opam<<endl;
} 
cout << endl; 
}

std::swap(s[i], s[i + 1]);