As confirmed by the OP via this comment, this is The Heir Chronicles series by Cinda Williams Chima.
The series is comprised of five main books, with the first having been published in 2006 and the fifth in 2014.
This excerpt from the first book, The Warrior Heir (2006), details the founding of five guilds by a quintet of cousins, who gained powers by swallowing magical stones stolen from a dragon's treasure. They didn't kill the dragon, but put it to sleep indefinitely with a sleeping potion:
The guilds were founded by five cousins who wandered into an enchanted valley in the North of England. There dwelt an immense dragon. The dragon slept atop a mountain made of precious jewels. The wanderers, upon discovering the treasure, and being unaware of the dragon, began chipping away pieces from it to carry away with them. The dragon awakened with a roar, demanding to know who dared steal his treasure. To save themselves, the cousins swallowed the stones they'd stolen. They were magical stones that conferred on them amazing powers, but also made them slaves to the dragon and tied them to the high valley known as Raven's Ghyll.
The cousins served the dragon for seven long years. At night, they conspired together, even though the dragon slept with one eye open. The wizard wrote a covenant of mutual protection that they all signed in blood. The soothsayer warned them that they must not kill the dragon, but only put it to sleep, or they would lose the powers they had acquired from the magical stones. The enchanter sang to the dragon, distracting it while the sorcerer brewed a powerful sleeping potion. To the warrior fell the task of pouring it in the dragon's ear.
The plan worked perfectly. It wasn't until the cousins were celebrating their victory over their erstwhile master that the wizard revealed that the covenant they had signed made wizards masters over the other guilds. If the covenant were broken, the dragon would wake and exact terrible vengeance on all of them.
Thus were founded the five guilds.
Here's the Goodreads synopsis for the first book, with focuses on Jack, a seemingly-ordinary teenager who learns that he's an heir to the warrior guild. The synopsis also mentions a magical tournament which Jack competes in:
An epic battle between good and evil...
Before he knew about the Roses, 16-year-old Jack lived an unremarkable life in the small Ohio town of Trinity. Only the medicine he has to take daily and the thick scar above his heart set him apart from the other high schoolers. Then one day Jack skips his medicine. Suddenly, he is stronger, fiercer, and more confident than ever before. And it feels great—until he loses control of his own strength and nearly kills another player during soccer team tryouts.
Soon, Jack learns the startling truth about himself: he is Weirlind, part of an underground society of magical people who live among us. At their helm sits the feuding houses of the Red Rose and the White Rose, whose power is determined by playing The Game—a magical tournament in which each house sponsors a warrior to fight to the death. The winning house rules the Weir.
As if his bizarre heritage isn't enough, Jack finds out that he's not just another member of Weirlind—he's one of the last of the warriors—at a time when both houses are scouting for a player.
According to the Wikipedia page for the series, Jack has a love interest in the book, Ellen, who's also a warrior heir, and competes in the same tournament:
Because of his Warrior status, Jack is wanted by both sides and encounters all sorts of problems involving capture by one of the Roses for entry in the Game, a contest with two warriors representing each Rose fighting to the death. Jack's assistant principal (who happens to be a wizard), Leander Hastings, trains Jack and wants him to join another group that he would enter. Jack's aunt Linda (an enchanter) doesn't want him to be in the Games at all and tries to protect him. Jack, while all of this is going on, is distracted by Ellen, a girl he is attracted to.
Ellen Stephenson: A rare Warrior(Weirlind), she fought in Tournament in the Raven's Ghyll; Jack Swift's equal partner on the battlefield and off.
The later books in the series focus on other heirs, which include a wizard heir, a dragon heir, and an enchanter heir.
