about the school, in general in Italy you don't need an authorization to open a kite school, because often are sporting association. if you want to kite starting from the beach you need an autoirzed kite zone (like you saw in porto pollo, la cinta, siniscola etc.), so usually the school ask to coastguard and to municipality set up a kite zone.
In punta trettu there isn't any authorized kite beach, and isn't possible to set up due for the channel for the boat, and the practice of kitesurf cannot stop the transit of boat in that area (sail boat, fisherman boats, pontoons and small yacth up to 25 mt).
about the question on the school that you saw..in my point of view they just start their job without checking/asking to coasguard about problem or restriction on the practice of kitesurf.
about your opinion that is the best spot in sardinia...it isn't very safe too shallow water. your portal is full of post about terrible accident in shallow water, and last august one guy had a displaced open fracture of the tibia and peroneal bone. he was almost to die for a heavy bleeding because the broken bone cut the calf.
About authorized kite zone in south sardinia there is ony Porto Botte, Porto Pino (from july 2014 - i set up this kite zone), Maddalena spiaggia in the city of Capoterra close to Cagliari.
from june end of september in sardinia and in general in Italy we can kite only starting from authorized kite zone.
it's better for everybody that want to spend a kite holiday in Sardinia to check with the local office of Coastguard (www.guardiacostiera.it
) if there is an authorized kite beach.
to avoid the problem and surprise that last year dozens of tourist, that went in holiday in punta trettu, got a fine from coastguard.
all the best