the function you posted is limited to 5 comments which means it should work, but anyway there is no need for a custom SQL query when you can use the native get_comments function
function showLatestComments() {
$comments = get_comments(array(
'status' => 'approve',
'number' => '5'
)
);
$output .= '<h2>Latest student perspectives <img src="https://wordpress.stackexchange.com/wp-content/themes/blue-and-grey/images/hmepage-tri.png" class="class3" /></h2>
<ul id="comm">';
foreach ($comments as $comment) {
$output .= '<li><strong>'. $comment->comment_author . ' said</strong> : "' . substr(strip_tags($comment->content),0,99). '..."</li>';
}
$output .= '</ul>';
echo $output;
}//end function