How can I test whether Eloquent model method is called by "creating" event in Laravel 5.4?

I would like to test whether a method is called when a eloquent event is triggered.

In the example below I would like to set the approved property automatically through isApproved() method when the Student instance is saved into the database.

Here is the Student model class:

<?php

namespace App;

use Illuminate\Database\Eloquent\Model;

class Student extends Model
{

    public $guarded = [];


    protected static function boot()
    {
        parent::boot();

        // set approved attribute if its value is NULL
        self::creating(function (self $student) {
            $student->approved = $student->approved ?? $student->isApproved();
        });
    }

    /**
     * @return bool
     */
    public function isApproved()
    {
        return ($this->age >= 14) && ($this->age <= 20);
    }
}

To achieve this i attached a callback function on the creating event for the Student class.

I am trying to test isApproved() method invocation with the following test:

<?php

namespace Tests\Feature;

use App\Student;
use Tests\TestCase;
use Illuminate\Foundation\Testing\DatabaseMigrations;

class StudentTest extends TestCase
{

    use DatabaseMigrations;

    /**
     * @test
     */
    public function student_is_auto_approved_on_save()
    {

        $mock = \Mockery::mock(Student::class);
        $mock->shouldReceive('isApproved')->once();

        Student::create([
            'name' => 'John Doe',
            'age' => 14
        ]);
    }
}

but the test does not pass and the following dump is shown

    PHPUnit 5.7.21 by Sebastian Bergmann and contributors.

E                                                                   1 / 1 (100%)

Time: 246 ms, Memory: 16.00MB

There was 1 error:

1) Tests\Feature\StudentTest::student_is_auto_approved_on_save
Mockery\Exception\InvalidCountException: Method isApproved() from Mockery_0_App_Student should be called
 exactly 1 times but called 0 times.

/students/vendor/mockery/mockery/library/Mockery/CountValidator/Exact.php:37
/students/vendor/mockery/mockery/library/Mockery/Expectation.php:298
/students/vendor/mockery/mockery/library/Mockery/ExpectationDirector.php:120
/students/vendor/mockery/mockery/library/Mockery/Container.php:297
/students/vendor/mockery/mockery/library/Mockery/Container.php:282
/students/vendor/mockery/mockery/library/Mockery.php:152
/students/vendor/laravel/framework/src/Illuminate/Foundation/Testing/TestCase.php:144
/home/user/.composer/vendor/phpunit/phpunit/src/TextUI/Command.php:186
/home/user/.composer/vendor/phpunit/phpunit/src/TextUI/Command.php:116

ERRORS!
Tests: 1, Assertions: 0, Errors: 1.

What I am doing wrong?