Something

SELECT p.nume, p.adresa
FROM eleviClase AS ec
INNER JOIN (

SELECT e2.cod AS cod, MAX( e2.medie ) , codClasa, e2.nume AS nume, e2.medie AS medie
FROM elevi AS e2
INNER JOIN eleviClase AS ec2 ON e2.cod = ec2.codElev
GROUP BY ec2.codClasa
) AS rez ON ec.codClasa = rez.codClasa
LEFT JOIN parinti AS p ON p.codElev = rez.cod
GROUP BY ec.codClasa
LIMIT 0 , 30