Why is my coroutine called after my normal function, even though my coroutine is called first?

Why is my co-routine called after my normal function, even though my co-routine is called first

I tried to call CheckClass() first then put the return value to the checkClassList, then I call my JoinClass() function, but no matter how I put it, the JoinCLass() is always called first.

This is my function:

public void OnJoinClass()
{
    StartCoroutine(checkClass(studentId));
    print(classCheckList.Length);
    JoinClass(studentId, InputClassCode.text);
    print("succes");
}

This is my CheckClass() co-routine:

IEnumerator checkClass(string id_Student)
{
    WWWForm form = new WWWForm();
    form.AddField("addStudentId", id_Student);

    WWW w = new WWW(URL5, form);

    yield return w;
    string classchecklist = w.text;

    classCheckList = classchecklist.Split(";");
    print(classCheckList.Length);
}

And this is the JoinCLass() function:

public void JoinClass(string id_Student, string id_Class)
{
    for (int i = 0; i < classCheckList.Length; i++)
    {
        print(classCheckList[i] + "==" + id_Class + "\n");
        if (classCheckList[i] == id_Class)
        {
            print(classCheckList[i]);
            isJoined = true;
        }
    }

    if (!isJoined)
    {
        WWWForm form = new WWWForm();
        form.AddField("addStudentId", id_Student);
        form.AddField("addClassId", id_Class);

        WWW www = new WWW(URL4, form);
    }
    else
    {
        print("Sudah Masuk Kelas");
    }
}

The first coroutine does get called first, but since you're yielding: yield return w, it will yield after the request has finished. Meanwhile, your code continues running synchronously and it starts another coroutine, which finishes before the WWW request is fulfilled.

If you're using only one coroutine, simply call JoinClass inside the coroutine, after it yields the result.

IEnumerator checkClass(string id_Student)
{
    WWWForm form = new WWWForm();
    form.AddField("addStudentId", id_Student);


    WWW w = new WWW(URL5, form);

    yield return w;
    string classchecklist = w.text;

    classCheckList = classchecklist.Split(";");
    print(classCheckList.Length);

    if (/* condition is met */) {
       JoinClass(studentId, InputClassCode.text);
    }
}

Alternatively, it is possible to yield a second coroutine inside the first one and remove the StartCoroutine call for the second one in the Start method.

Check my answer here on how to handle "master" and "slave" coroutines:
https://stackoverflow.com/a/76374713/7131671

Be aware of that, you call "JoinClass" method right after starting "checkClass" coroutine. But "checkClass" coroutine uses "yield" and it may take a little time to complete (certainly will not complete immediately).

So you should call "JoinClass" after being sure that "checkClass" has completed. Maybe you can call JoinClass inside checkClass method like that.

public void OnJoinClass()
{
    StartCoroutine(checkClass(studentId));
}

IEnumerator checkClass(string id_Student)
{
    WWWForm form = new WWWForm();
    form.AddField("addStudentId", id_Student);


    WWW w = new WWW(URL5, form);

    yield return w;
    string classchecklist = w.text;

    classCheckList = classchecklist.Split(";");
    print(classCheckList.Length);

    JoinClass(studentId, InputClassCode.text);
    print("succes");
}



public void JoinClass(string id_Student, string id_Class)
{
    for (int i = 0; i < classCheckList.Length; i++)
    {
        print(classCheckList[i] + "==" + id_Class + "\n");
        if (classCheckList[i] == id_Class)
        {
            print(classCheckList[i]);
            isJoined = true;
        }
    }

    if (!isJoined)
    {
        WWWForm form = new WWWForm();
        form.AddField("addStudentId", id_Student);
        form.AddField("addClassId", id_Class);


        WWW www = new WWW(URL4, form);
    }
    else
    {
        print("Sudah Masuk Kelas");
    }
    
}