朋友功能奇怪的行为

我正在自学c++，我编写的其中一个程序是一种学校信息系统，它不是我第一个面向对象的程序，但这次我经历了一个朋友函数的一些奇怪行为。我有一个派生类Student，它有一个私有的Courses对象的静态成员数组，最后是主容器类Class，它有一个Students对象的动态成员数组。

你可以看到student的avg成员是使用Courses类的朋友函数计算的(calc_avg接收Courses的数组和它的成绩并除以课程号)，它像一个魅力。问题一旦我开始试着计算平均成绩(class_avg) Class对象——我使用相同的逻辑——创建一个朋友Student的函数(class_avarage将可以访问每个学生avg),将获得Student对象数组的大小,增加每个学生平均成绩和除以数组大小,出于某种原因,它接收并返回垃圾值,我试图修复它已经3个小时,我不能找到这个问题,帮助我，如果你可以-这里是相关的代码(不包括独立的派生类和输出函数被最小化):

main//仅包含容器对象创建和IO函数(包括所有.h文件)

Class test1;
test1.input_class();
test1.output_class();

Student.h

#include "Person.h"
#include "Courses.h"
#ifndef STUDENT_H
#define STUDENT_H
class Student : public Person
{
public:
    Student();
    void input_student ();
    void output_student ();
    friend float class_avarage (Student* array, int);
    ~Student();
protected:
private:
    Courses my_courses [6];
    float avg;
};
#endif // STUDENT_H

Student.cpp

#include "Student.h"
Student::Student()
{
//ctor
}
void Student::input_student ()
{
cout << "Please enter students info:" << endl;
input_info();
my_courses[0].input_grades ("Math");
my_courses[1].input_grades ("English");
my_courses[2].input_grades ("Science");
my_courses[3].input_grades ("History");
my_courses[4].input_grades ("Art");
my_courses[5].input_grades ("Sports");
}
void Student::output_student ()
{
int i;
cout << "Students info:" << endl;
output_info();
cout << "   Course  Grade" << endl;
for (i = 0; i < 6; i++)
{
    my_courses[i].output_grades ();
}
avg = calc_avg(my_courses, 6);
cout << "   " << "Average grade for student: " << avg << endl;
}
Student::~Student()
{
//dtor
}
float calc_avg (Courses* array, int size)
{
int i;
float sum = 0;
for (i = 0; i < size; i++)
{
    sum += array[i].grade;
}
return sum/size;
}

Courses.h

#include <iostream>
#include <iomanip>
#include <string>
#ifndef COURSES_H
#define COURSES_H
using std::cout;
using std::cin;
using std::endl;
using std::string;
class Courses
{
public:
    Courses();
    void input_grades(string);
    void output_grades();
    friend float calc_avg (Courses*, int);
    ~Courses();
protected:
private:
    string course_name;
    int grade;
};
#endif // COURSES_H

Courses.cpp

#include "Courses.h"
Courses::Courses() : course_name ("Default"), grade (-1)
{
//ctor
}
void Courses::input_grades (string temp_name)
{
course_name = temp_name;
cout << "Enter " << temp_name << " grade: ";
cin >> grade;
cin.get();
}
void Courses::output_grades ()
{
cout << "   " << course_name << ":    " << grade << endl;
}
Courses::~Courses()
{
//dtor
}

Class.h

#include <iostream>
#include <string>
#include "Teacher.h"
#include "Student.h"
#ifndef CLASS_H
#define CLASS_H
using std::cout;
using std::cin;
using std::endl;
using std::string;
class Class
{
public:
    Class();
    void input_class ();
    void output_class ();
    ~Class();
protected:
private:
    string class_name;
    Teacher class_teacher;
    Student* pupils;
    int students_number;
    float class_avg;
};
#endif // CLASS_H

Class.cpp

#include "Class.h"
Class::Class() : class_name ("A"), pupils (NULL), students_number (0), class_avg (0)
{
//ctor
}
void Class::input_class ()
{
int i;
//input class data
cin >> students_number;
cin.get();
pupils = new (std::nothrow) Student [students_number];
if (!pupils)
{
    cout << "Allocation failed!" << endl;
}
else
{
    for (i = 0; i < students_number; i++)
    {
        pupils[i].input_student();
    }
}
class_avg = class_avarage (pupils, students_number);
}
void Class::output_class ()
{
int i;
cout << "   Displaying info of class :" << class_name << endl <<
    "   Class average grade: " << class_avg << endl;
class_teacher.output_teacher();
cout << "   Number of students: " << students_number << endl << "   Students: " << endl;
for (i = 0; i < students_number; i++)
{
    pupils[i].output_student();
}
}
Class::~Class()
{
//dtor
}
float class_avarage (Student* array, int size)
{
cout << size << endl;
int i;
float total = 0;
for (i = 0; i < size; i++)
{
    total += array[i].avg;
}
return total/size;
}