wp_ajax returns success but data is null

I am creating a plugin that has two select menus. When the user selects the first option the second select should populate. This form is placed within a widget.

I am attempting to use wp_ajax_. The code for the form is in a class:

class CourseSearch {

	public function __construct( $plugin ) {

		add_action( 'wp_ajax_subject_search', array( $this, 'return_subjects') );
    	add_action( 'wp_ajax_nopriv_subject_search', array( $this, 'return_subjects') );
	}

	function return_subjects() {

	    $subjects = $this->get_subjects( $_POST['grade'] );

	    echo json_encode( $subjects );
	}
}

Javascript:

$('#select_grade').on( 'change', function( event ) {

		display_subjects( event );
	});

/**	Functions
------------------------------------------------------------------------*/	

	function display_subjects( event ) {

		var grade = event.currentTarget.value;
		var path  = plugin.path + 'classes/class-course-search.php';

		console.log( path, grade );

		$.ajax({
			url: path,
			type: 'post',
			dataType: 'json',
			data: {
				grade:  grade,
				action: 'subject_search'
			},
			success: populate_subject_list,
			error: clear_subject_list,
		});
	}

	function populate_subject_list( data ) {

		console.log( 'success', this, data );
	}

	function clear_subject_list( data ) {

		console.log( 'fail', data );
	}

I am at a dead end, so any help is appreciated.