<?php
$termins = mysqli_query($link,"
            SELECT id,class,
            GROUP_CONCAT(termin SEPARATOR ', ') AS termin
            FROM termins
            GROUP BY class
            ORDER BY class");
        while($row = mysqli_fetch_assoc($termins)){

?>
<h3><?php echo htmlspecialchars($row['class']);?></h3>
<a href="#<?php echo htmlspecialchars($row['id']);?>"><?php echo htmlspecialchars($row['termin']);?></a>
<br>

<?php
}
?>