MySQL – Data Model for Online Examination Matching Questions

I'm designing a module for a client that allows them to administer certification examinations via their website to seminar attendees. I'm stuck on how to store the data for matching questions and keep the questions and answers in different tables.

First, if my data model doesn't make sense, please let me know. If there is a more elegant way to think about the problem, I want to know. What follows is part of the install method for the module, for the purpose of showing the additional tables and their columns.

    //Create test table
    $this->db->query("CREATE TABLE IF NOT EXISTS `" . DB_PREFIX . "test` (
      `test_id` int(11) NOT NULL AUTO_INCREMENT,
      `title` varchar(255) NOT NULL,
      `percent_to_pass` int(11) NOT NULL,   
      `attempts_allowed` int(11) NOT NULL,
      `time_limit_seconds` int(11) NOT NULL,
      `date_available` datetime NOT NULL,
      PRIMARY KEY (`test_id`)
    ) ENGINE=MyISAM DEFAULT CHARSET=utf8 AUTO_INCREMENT=1 ;");

    //Create products_to_test table
    $this->db->query("CREATE TABLE IF NOT EXISTS `" . DB_PREFIX . "product_to_test` (
      `product_to_test_id` int(11) NOT NULL AUTO_INCREMENT,
      `product_id` int(11) NOT NULL,
      `test_id` int(11) NOT NULL,
      PRIMARY KEY (`product_to_test_id`)
    ) ENGINE=MyISAM DEFAULT CHARSET=utf8 AUTO_INCREMENT=1 ;");

    //Create customer_to_test table
    $this->db->query("CREATE TABLE IF NOT EXISTS `" . DB_PREFIX . "customer_to_test` (
      `customer_to_test_id` int(11) NOT NULL AUTO_INCREMENT,
      `customer_id` int(11) NOT NULL,
      `test_id` int(11) NOT NULL,
      `attempts` int(11) NOT NULL, //Number of attempts at the exam
      `attempts_allowed` int(11) NOT NULL, //Will usually be 1, but will be incremented in the instance that the test is reset, allowing for multiple attempts upon approval.
      `percent_correct` int(11) NOT NULL, //Percentage of answers correct
      `responses` varchar(255) NOT NULL, //Serialized list of responses (question_id,answer_id;question_id,answer_id;
      `question_order` varchar(255) NOT NULL, //Serialized list of the questions/answers in the order presented for this exam (question_id,answer_1_id,answer_n_id,...;etc.)
      `passed` tinyint(1) NULL,  //True/False - Did the customer pass the exam?
      `date_taken` datetime NULL,  //Date and Time the exam was taken.
      PRIMARY KEY (`customer_to_test_id`)
    ) ENGINE=MyISAM DEFAULT CHARSET=utf8 AUTO_INCREMENT=1 ;");

    //Create test_questions table
    $this->db->query("CREATE TABLE IF NOT EXISTS `" . DB_PREFIX . "test_questions` (
      `question_id` int(11) NOT NULL AUTO_INCREMENT,
      `question_type` int(2) NOT NULL, //Indexed to question types: 1=Multiple Choice, 2=True/False, 3=Matching, 4=Short Answer/Fill in the Blank, 5=Essay
      `question_stem` varchar(255) NOT NULL, //If a matching question, then the stems will be serialized.
      `pre_question_stem_text` varchar(255) NULL,  //For text that applies to multiple questions, i.e. 101-104: Select the most appropriate graph
      `pre_question_stem_span` int(2) NULL, //Indicates how many questions the pre-question stem text should span (for presentation/indentation purposes)
      PRIMARY KEY (`question_id`)
    ) ENGINE=MyISAM DEFAULT CHARSET=utf8 AUTO_INCREMENT=1 ;");

    //Create question_to_test table
    $this->db->query("CREATE TABLE IF NOT EXISTS `" . DB_PREFIX . "question_to_test` (
      `question_to_test_id` int(11) NOT NULL AUTO_INCREMENT,
      `test_id` int(11) NOT NULL,
      `question_id` int(11) NOT NULL,
      PRIMARY KEY (`question_to_test_id`)
    ) ENGINE=MyISAM DEFAULT CHARSET=utf8 AUTO_INCREMENT=1 ;");

    //Create test_answers table
    $this->db->query("CREATE TABLE IF NOT EXISTS `" . DB_PREFIX . "test_answers` (
      `answer_id` int(11) NOT NULL AUTO_INCREMENT,
      `question_id` int(11) NOT NULL,
      `answer_text` varchar(255) NULL, //If a matching question, this will contain a serialized list of the answer choices. (choice_id,text;etc.)
      `answer_img` varchar(255) NULL,
      `correct` tinyint(1) NOT NULL,
      `correct_pairs` varchar(255) NULL, //If a matching question, then this will contain a serialized list of the correct pairs to compare the responses to.
      PRIMARY KEY (`answer_id`)
    ) ENGINE=MyISAM DEFAULT CHARSET=utf8 AUTO_INCREMENT=1 ;");

    //Create answer_to_question table
    $this->db->query("CREATE TABLE IF NOT EXISTS `" . DB_PREFIX . "answer_to_question` (
      `answer_to_question_id` int(11) NOT NULL AUTO_INCREMENT,
      `question_id` int(11) NOT NULL,
      `answer_id` int(11) NOT NULL,
      PRIMARY KEY (`answer_to_question_id`)
    ) ENGINE=MyISAM DEFAULT CHARSET=utf8 AUTO_INCREMENT=1 ;");

Now, this makes sense to me for multiple choice, true/false, and I can even make it work for essay questions. It feels like a lot of tables to accomplish something so simple, but it does have the advantage of keeping all the parts of the exam separate and easy to visualize. By serializing certain data depending on the question type, it works.

I've decided to serialize the stems and the answers for matching questions and store them inside the existing tables, and have added an additional field to the answers table to store the correct pairs, which can be read back when the exam is being graded.

I'm still interested in hearing thoughts from those with more experience. Thank you for your time and effort. I'm new to data design, and appreciate the help.