Even though messaging is disabled, teachers can still send a message to their selected users in the participants block.

Change this:
$displaylist['messageselect.php'] = get_string('messageselectadd');

To this:
if ((has_capability('moodle/course:bulkmessaging', $context) && $context->id != $frontpagectx->id) and $CFG->messaging)

To require that they have the capability AND messaging is enabled sitewide.