Frontend Form - Checkboxes cannot "all" be unchecked
Your code is correct but you need to add another line near the top:

 &lt;input type="hidden" name="fields[volunteerPositions]" value=""&gt;


From the documentation:


  If you are including a Checkboxes field on a front-end entry from, make sure to include a hidden input before the checkboxes, so that an empty value is submitted if no checkboxes are checked.