Defining class in header file and function in separate file

Just my practice. I/O a student's information.

//Header file(student.h)
#ifndef STUDENT_H_INCLUDED
#define STUDENT_H_INCLUDED
#include <string>

using namespace std;

class Student
{
    public:
        int outputStudentInfo();
        int inputStudentInfo();
    private:
        int studentIDNumber;
        string studentName;
        char studentSex;
};

#endif // STUDENT_H_INCLUDED

//Class functions file(studentFunctions.cpp)
#include <iostream>
#include "student.h"

using namespace std;

int Student::inputStudentInfo()
{
    cin >> studentIDNumber;
    cin >> studentName;
    cin >> studentSex;
    return 0;
}

int Student::outputStudentInfo()
{
    cout << "Information you entered is:" <<endl;
    cout << "ID:" << studentIDNumber << endl;
    cout << "Name:" << studentName << endl;
    cout << "Sex:" << studentSex << endl;
    return 0;
}

//main.cpp
#include <ostream>
#include "student.h"

using namespace std;

int main()
{
    Student student;
    student.inputStudentInfo();
    student.outputStudentInfo();
    return 0;
}