Add/Remove class if checkbox is checked/unchecked (AngularJS)

I need to toggle a class if a checkbox is checked/unchecked with AngularJS. My Code is working on Plunker but in my controller there is no functionality.

Here is the Plunker: Click!

My Controller looks like this:

// initialize iCheck
angular.element('.i-checks').iCheck({
  checkboxClass: 'icheckbox_square-green',
  radioClass: 'iradio_square-green'
});

//Checkbox functionality
$scope.isChecked = function() {
    if ($scope.checked) {
      return false;
    }else{
      return true;
    }
};

HTML

          <div class="panel panel-default confirm-panel accept-terminplanung">
            <div class="panel-body text-muted ibox-heading">
              <form action="#" method="post" role="form">
                <div class="i-checks" iCheck>
                  <label class="">
                    <div class="icheckbox_square-green">
                      <input type="checkbox" class="i-checks" ng-model="checked">
                    </div>
                      <p class="no-margins">Hiermit  akzeptiere ich  die Bedingungen des geschlossenen Dozentenvertrages und nehme den Bildungsauftrag für die ausgewählten Ausbildungen verbindlich an.</p>
                  </label>
                </div>
                <button type="button" name="termin-bestaetigen" class="btn btn-w-m btn-warning submit-appointment disabled" ng-class="{disabled: isChecked()}">Terminplanung übernehmen</button>
              </form>
            </div>
          </div>

What am I doing wrong?