I think this is the best diving school on Koh Tao. Not too many people in a class, good instructions, nice people.
But diving in Koh Tao was very disappointing compared to other diving spots I have been.
Viz was bad, not so many interesting sealife (only the usual suspects). The thing I liked the least was that tourism has ruined this island. Only bars with loud music, drunken English people in the evening. During days we weren't diving it was almost impossible to find a beach for swimming.
So...diving school is the best choice if you decide to go to Koh Tao! But I wouldn't recomment Koh Tao.