You can use 2 ways:
1) Create a custom class and rewrite function label in it.
Because just function label return html string php <label>...</label>
.
2) Use another construction, like :
echo '<label for="', $element->getName(), '">', $element->getLabel(), '</label>';
or
printf('<label for="%s" class="%s">%s</label>',$element->getName(),$your_class_for_label,$element->getLabel());
in render form function or in a view.