Выводит в файл только последнего студента на C++

#ifndef power_h
#define power_h
#include <fstream>
#include <string>
 
using namespace std;
 
class Student {
private:
    int age;
    string name;
    string patronymic;
    string surname;
    string phone;
    string group;
public:
    void powerAll(int age, string name, string surname) {
        this->age = age;
        this->name = name;
        this->surname = surname;
    }
    
    void powerPrintAll() {
        ofstream output_file("rezult.txt", ios::out);
        output_file << this->name << " " << this->surname << ". Возраст: " << this->age << endl;
        output_file.close();
        
        cout << this->name << " " << this->surname << ". Возраст: " << this->age << endl;
    }
};
 
#endif /* power_h */
//cout << this->name << " " << this->surname << ". Возраст: " << this->age << endl;

#include <iostream>
#include <clocale>
#include "power.h"
 
using namespace std;
 
int main() {
    setlocale(LC_ALL, "Russian");
    
    Student martin;
    martin.powerAll(38, "Martin", "McFly");
    martin.powerPrintAll();
    
    Student john;
    john.powerAll(175, "John", "Silver");
    john.powerPrintAll();
    
    return 0;
}

ofstream output_file("rezult.txt", ios::app);