php数据库表创建失败

我使用以下要点进行OOP尝试来创建数据库连接：

https://gist.github.com/jonashansen229/4534794

到目前为止，它似乎有效。

但是数据库表passed_exels的创建失败。

编辑：

在最近的评论和建议之后，我更新了我的代码：

require_once 'Database.php'; // the gist 4534794
class DatabaseSchema {
  public function createStudents() {
    $db = Database::getInstance();
    $mysqli = $db->getConnection();
    $create_students = 'CREATE TABLE IF NOT EXISTS students (
      id INT(6) UNSIGNED NOT NULL AUTO_INCREMENT PRIMARY KEY,
      firstname VARCHAR(40) NOT NULL,
      lastname VARCHAR(40) NOT NULL,
      university VARCHAR(50)
    )';
    $result = $mysqli->query($create_students);
  }
  public function createPassedExams() {
    $db = Database::getInstance();
    $mysqli = $db->getConnection();
    $create_passed_exams = 'CREATE TABLE IF NOT EXISTS passed_exams (
      id INT(6) UNSIGNED NOT NULL AUTO_INCREMENT PRIMARY KEY,
      name VARCHAR(40) NOT NULL,
      student_id INT(6),
      FOREIGN KEY (student_id) REFERENCES students(id) ON DELETE CASCADE
    )';
    $result = $mysqli->query($create_passed_exams);
  }
}
$db_student = new DatabaseSchema();
$db_student->createStudents();
$db_student->createPassedExams();

当我在mysql控制台中查看时，只创建了表学生。为什么缺少passed_exels表？