The Thunder Egg (Dragon Games Book #1) by Maddy Mara
One fateful afternoon, classmates Luca, Yazmine and Zane stumble upon a strange-looking rock called a geode in one of the rooms at school. To their surprise, a touch of this geode transports them to a magical land named Imperia.
The geode is actually a dragon egg! It’s one of three Thunder Eggs that need to be returned home to secret locations in Imperia. Once these eggs hatch, the dragons born from them will restore peace to the land. In order to return the first egg, the kids must face off against the evil ruler of Imperia in a dangerous game.