Rick Riordan, The Lost Hero (2010).
JASON HAS A PROBLEM. He doesn’t remember anything before waking up in a bus full of kids on a field trip. Apparently he has a girlfriend named Piper, and his best friend is a guy named Leo. They’re all students at the Wilderness School, a boarding school for “bad kids,” as Leo puts it. What did Jason do to end up here? And where is here, exactly? Jason doesn’t know anything—except that everything seems very wrong.
PIPER HAS A SECRET. Her father has been missing for three days, ever since she had that terrifying nightmare about his being in trouble. Piper doesn’t understand her dream, or why her boyfriend suddenly doesn’t recognize her. When a freak storm hits during the school trip, unleashing strange creatures and whisking her, Jason, and Leo away to someplace called Camp Half-Blood, she has a feeling she’s going to find out, whether she wants to or not.
LEO HAS A WAY WITH TOOLS. When he sees his cabin at Camp Half-Blood, filled with power tools and machine parts, he feels right at home. But there’s weird stuff, too—like the curse everyone keeps talking about, and some camper who’s gone missing. Weirdest of all, his bunkmates insist that each of them—including Leo—is related to a god. Does this have anything to do with Jason’s amnesia, or the fact that Leo keeps seeing ghosts?”
Ally Carter, Out of Sight, Out of Time (2012).
Gallagher Girls #5.
The last thing Cammie Morgan remembers is leaving the Gallagher Academy to protect her friends and family from the Circle of Cavan-an ancient terrorist organization that has been hunting her for over a year. But when Cammie wakes up in an alpine convent and discovers that months have passed, she must face the fact that her memory is now a black hole. The only traces left of Cammie’s summer vacation are the bruises on her body and the dirt under her nails, and all she wants is to go home.
Once she returns to school, however, Cammie realizes that even the Gallagher Academy now holds more questions than answers. Cammie, her friends, and mysterious spy-guy Zach must face their most difficult challenge yet as they travel to the other side of the world, hoping to piece together the clues that Cammie left behind. It’s a race against time. The Circle is hot on their trail and willing stop at nothing to prevent Cammie from remembering what she did last summer.”
Patricia McKillip, The Riddle-Master of Hed (1976).
Long ago, the wizards had vanished from the world, and all knowledge was left hidden in riddles. Morgon, prince of the simple farmers of Hed, proved himself a master of such riddles when he staked his life to win a crown from the dead Lord of Aum. But now ancient, evil forces were threatening him. Shape changers began replacing friends until no man could be trusted. So Morgon was forced to flee to hostile kingdoms, seeking the High One who ruled from mysterious Erlenstar Mountain. Beside him went Deth, the High One’s Harper. Ahead lay strange encounters and terrifying adventures. And with him always was the greatest of unsolved riddles; the nature of the three stars on his forehead that seemed to drive him toward his ultimate destiny.”