Javascript contact form validation code
Here is a suggestion:

var tosCheckbox = $('.tos-checkbox');
var checkBoxValidation = (function(label, checkbox, errorDisplay) {
    var checked = checkbox.is(':checked');
    return function(e) {
        if (e.type == 'submit' &amp;&amp; !checked) e.preventDefault();
        tosLabel.toggleClass('tos-is-checked', checked);
        formError.toggleClass('tos-error', !checked);
    }
})(tosLabel.closest('label'), tosCheckbox, formError.closest('label').find('.form-error'));

tosCheckbox.change(checkBoxValidation);
tosLabel.closest('form').submit(checkBoxValidation);


Baiscally I tried to make the code dependent/related to tosCheckbox, finding both label, error and form from that element. This avoids pointing to the wrong thing like '.form-error' which I see more than one in the page. This way only the one inside the label will receive the class.

DRYed also the event listener callbacks, the class add/remove and added a check to prevent the form submit event. Not sure you need this, but its a heads up in case you are not stoping the submit in case of error.

Hope it helps.