jQuery validator is not working for checkbox lists when using CSS class to validate

I have some check boxes like so:

<input class="mandatory" name="Notes8~1" id="Notes8~1" type="checkbox" value="Yes">Option 1<br>
<input class="mandatory" name="Notes8~2" id="Notes8~2" type="checkbox" value="Yes">Option 2<br>
<input class="mandatory" name="Notes8~3" id="Notes8~3" type="checkbox" value="Yes">Option 3<br>

(yes I know the <input> tags are not closed, maybe I need to fix that!)

And I want to require that the user checks at least one of them. So I have this JavaScript:

jQuery.ready({function() {jQuery.validator.addClassRules('mandatory', {required: true});}});

to add validation to the check boxes. However this doesn't work; from stepping through the code it looks like the validation code is executed but it somehow passes even if all the check boxes are blank. How can I require the user to check at least one box? I was able to do it by giving them all the same name Notes8 but that won't work in this app because other code is relying on them having unique names.