The Schools of Great Warriors were formed by the honorable three warriors of the Hand of Flame; O'gat, Kollus and To'Kar. It was created after the defeat of the tyrant Molor when the first Emperor of the Klingon Empire Kahless the Unforgettable honored the three warriors by asking them to train future generations in the art of honorable combat.
There were three such schools each of which taught its warriors the wisdom of Kahless as well as the combat style of the school's founder. For generatons, Klingons after the Rite of Ascension sought entry into the school in order to become members of the elite Honor Guard with only the best being bestowed his highest of honors. (TNG video game: Klingon Honor Guard)