You could do something like this and just add the classes to the call in your theme:
<?php
// vars
$field = get_field_object('your_select_field');
$value = $field['value'];
$label = $field['choices'][ $value ];
?>
<p>Items: <span class="select2-results__option item-<?php echo $value; ?> <?php echo $value; ?>"><?php echo $label; ?></span></p>
You can add whatever default classes you need and then the particular select option item classes programmatically via the field object.
Does that help?
