required field for checkboxes

I have added list of checkboxes in following way. And I have added required property also. Because user should select at least one checkbox.

<div ng-repeat="student in vm.students">
    <label class="checkbox-inline">
        <input type="checkbox" value="{{studentName}}" ng-model="student.selected" name="students" required>
        {{sim.name}}
    </label>
</div>

But this one doesn't work. It works for last checkbox only. If check and uncheck the last one the error message is appear. Any possible way will be highly appreciable.