prova questa:
codice:
select cl.ext,nome,cognome from classi cl
inner join codice 
on cl.ext = codice.ext 
where cl.ext not like "zz%" group by concat(cl.ext,nome,cognome)
having count(concat(cl.ext,nome,cognome)) < 2 and
cl.ext in 
(select ext from classi group by ext having count(ext) > 1);