Stormbringer: The Predator's Stone by G. R. Boden

Abandoned at birth and shuffled through the system all her life, Cinder is on the verge of being expelled from yet another school. Despite high test scores, Cinder’s records show a penchant for fighting, defiance, impulsivity, and “an unwillingness to develop appropriate decision-making skills”―whatever that means. When a lunch-time brawl breaks out in the cafeteria, Cinder gloomily prepares for what has become all-too familiar. This fight, however, won’t end like all the others. Allfather has dispatched his Predators, an elite team of bodyguards, to bring Cinder to the Proving Grounds, where she will be broken, disciplined, trained, and given her quest directive. Cinder may be in an alternate realm full of Valkyries, dark elves, and world-encircling serpents, but she still has a chip on her shoulder the size of Thor’s hammer, and she’s not about to let Odin or any other god push her around.