Thank you all for your help and suggestions. Each had a perspective that ultimately helped me to arrive at a successful end, and I appreciate it very much!
This is what I finally settled on. After playing around with all the differing solutions. I started thinking with logic and finally the following dawned on me:
<div id="inp_school_type" class="form-group col" is="dmx-checkbox-group">
<legend class="pb-2" style="font-size: 16px;">School Type</legend>
<!-- group checkboxes -->
<div class="form-check form-check-inline mr-5">
<input class="form-check-input" type="checkbox" id="inlineCheckboxBoarding" value="Boarding" checked>
<label class="form-check-label" for="inlineCheckboxBoarding">Boarding</label>
</div>
<div class="form-check form-check-inline mr-5">
<input class="form-check-input" type="checkbox" id="inlineCheckboxDay" value="Day" checked>
<label class="form-check-label" for="inlineCheckboxDay">Day</label>
</div>
<div class="form-check form-check-inline mr-0">
<input class="form-check-input" type="checkbox" id="inlineCheckboxNight" value="Night" checked>
<label class="form-check-label" for="inlineCheckboxNight">Night</label>
</div>
<!-- group checkboxes -->
<!-- {{inlineCheckboxBoarding.checked.then('Boarding', '')+' '+inlineCheckboxDay.checked.then('Day', '')+' '+inlineCheckboxNight.checked.then('Night', '')}} -->
</div>
<input type="hidden" name="school_type" dmx-bind:value="(inlineCheckboxBoarding.checked.then(' Boarding ', '')+' '+inlineCheckboxDay.checked.then(' Day ', '')+' '+inlineCheckboxNight.checked.then(' Night ', ''))" id="school_type">
It works perfectly.
