why 'Art' Index not added in subjects Array

<?php

class Student {

    private $name;
    private $age;
    private $score;
    private $subjects = array (
        'Arabic'    => 0 ,
        'English'   => 0 ,
        'Math'      => 0 ,
        'Science'   => 0
    );
    private $minScore = 150;
    private $maxScore = 300;

    public function __construct($name , $age)
    {
        $this->setName($name);
        $this->setAge($age);
    }

    private function setName($name)
    {
        $name = strtolower($name);
        $name = filter_var($name,FILTER_SANITIZE_STRING);
        $name = ucwords($name);
        $name = substr($name, 0 , 30);
        $this->name = $name;
    }

    public function getName() {
        return $this->name;
    }

    private function setAge($age)
    {
        $age = filter_var($age, FILTER_SANITIZE_NUMBER_INT);
        $age = abs($age);

        if ($age <= 6 || $age  >= 18) {
            throw new Exception("Error");
        } else {
            $this->age = $age;
        }

    }

    public function getAge() {
        return $this->age;
    }

    public function getSubject() {
        return $this->subjects;
    }

    public function getMinScore() {
        return $this->minScore;
    }

     public function getMaxScore() {
        return $this->maxScore;
    }

}

class Grade1Student extends Student
{



    public function __construct($name,$age)
    {
        parent::__construct($name,$age);
        $this->minScore=180;
        $this->maxScore=400;
        $this->subjects['Art'] = 0;
    }
}

$mohamed = new Grade1Student('Mohamed' , 12);
print_r($mohamed->getSubject());