Not correctly passing value of checked box-html form (with javascript)

I have worked on this for a while and gave up because I couldn't figure it out, but now I'm back to it. I have an email newsletter signup form on my website, and I have a checkbox for people to check if they are interested in music classes with me. The way I want the form to work is that if they are not interested in classes, I will get no value, but if they are interested in classes I get "Interested in classes - $city". Right now I am getting "Interested in classes - $city" in both cases.

The form is kind of complicated because I'm using javascript as well as the regular html code (hiding divs, etc), so I think I'm getting confused there.

The form:

<form action="../index_success.php" method="post" id="sendEmail" class="email">
            <h3 class="register2">Newsletter Signup:</h3>
            <ul class="forms email">
                <li class="name">
                    <label for="yourName">Name: </label>
                    <input type="text" name="yourName" class="info" id="yourName" value="<?php echo $_POST['yourName']; ?>" /><br />
                </li>
                <li class="city"><label for="yourCity">City: </label>
                    <input type="text" name="yourCity" class="info" id="yourCity" value="<?php echo $_POST['yourCity']; ?>" /><br />
                </li>
                <li class="classes"><label for="classInterest">Interested in classes?: </label>
                    <input type="checkbox" name="classInterest" class="info" id="classInterest" value="Yes" /><br />
                </li>
                <li class="email">
                    <label for="emailFrom">Email: </label>
                    <input type="text" name="emailFrom" class="info" id="emailFrom" value="<?php echo $_POST['emailFrom']; ?>" />
                     <?php if(isset($emailFromError)) echo '<span class="error">'.$emailFromError.'</span>';

                     ?>
                </li>
                <li class="buttons email">
                     <button type="submit" id="submit">Send</button>
                     <input type="hidden" name="submitted" id="submitted" value="true" />
                </li>
            </ul>
        </form>

The javascript-my apologies if this code is not relevant to the question-I'm still pretty new so I have found that it is best to include too much rather than not enough! (I'm using jQuery):

    <script type="text/javascript">
$(document).ready(function(){

 $('#emailFrom')
  .focus(function(){
   if ($('#overlay').length) { return; } // don't keep adding overlays if one exists
   $('#sendEmail')

    .find('.name, .city, .classes').slideDown(300, function(){ $(this).show(); });
   $('.outeremailcontainer').css({ position: 'relative', bottom: 0, left: 0, zIndex : 1001 });
   $('<div id="overlay"></div>').appendTo('body');
  });

 $('#overlay').live('click', function(){
   $('#sendEmail')
    .css({ backgroundColor : 'transparent' })
    .find('.name, .city, .classes').slideUp(300);
   $('.outeremailcontainer').css({ position : 'static' });
   $('#overlay').remove();
  });

});
</script>

Here is the javascript for checking the errors on the form and emailing setting the value for the class interest classInterestVal:

$(document).ready(function(){
  $("#submit").click(function(){
    $(".error").hide();
    var hasError = false;
    var emailReg = /^([\w-\.]+@([\w-]+\.)+[\w-]{2,4})?$/;


    var emailFromVal = $("#emailFrom").val();

    if(emailFromVal == '') {
      $("#emailFrom").after('<span class="error"><br />You forgot to enter the email address to send from.</span>');
      hasError = true;

    } else if(!emailReg.test(emailFromVal)) {
      $("#emailFrom").after('<span class="error"<br />>Enter a valid email address to send from.</span>');
      hasError = true;
    }

    var yourNameVal = $("#yourName").val();
    if(yourNameVal == '') {
        $("#yourName").after('<span class="error"><br />You forgot to enter your name.</span>');
        hasError = true;
    }

    var yourCityVal = $("#yourCity").val();
    if(yourCityVal == '') {
        $("#yourCity").after('<span class="error"><br />You forgot to enter your city.</span>');
        hasError = true;
    }
    var classInterestVal = $("#classInterest").val();


    if(hasError == false) {
      $(this).hide();
      $("#sendEmail li.buttons").append('<img src="/wp-content/themes/default/images/template/loading.gif" alt="Loading" id="loading" />');
      $.post("/includes/sendemail.php",
//emailTo: emailToVal, 
           { emailFrom: emailFromVal, yourName: yourNameVal, yourCity: yourCityVal, classInterest: classInterestVal },
             function(data){
            $("#sendEmail").slideUp("normal", function() {
              $("#sendEmail").before('<h3 class="register2">Thank you!  You\'re on the email list!</h3><p class="emailbox">Click <a href="http://rattletree.com/Songs/Live_EP/Chikende.mp3">HERE</a> for your free song.</p>');
            });
             }
         );
    }
    return false;
  });
});

And lastly here is the sendEmail.php (this is where I put the conditional for $classInterest):

    <?php 
$mailTo = '[email protected]'; // This is the hardcoded Recipient Address 
$mailSubject = 'Newsletter'; // This is the hardcoded Subject
$mailFrom = $_POST['emailFrom']; 
$yourName = $_POST['yourName']; 
$yourCity = $_POST['yourCity'];


if (isset($_POST['classInterest']) && $_POST['classInterest'] == 'Yes'){
                            $classInterest ="Class Interest - " . $yourCity;
                        }
                        else {
                            $classInterest = '';
                        }
$mailHeader = "From: {$mailFrom}"; 
$mailBody = "Name = {$yourName} City = {$yourCity} Class interest = {$classInterest}";

mail( $mailTo , $mailSubject , $mailBody , $mailHeader );
?>