Titans

Titans is a sequel to the Pegasus series. The Titans and Olympians are at peace. the kids are all going to school. The book is much more lighthearted than the Pegasus books. It also as written intentionally as a multipart series. This book ends on a cliffhanger with nothing wrapped up. 

Random beings, including humans start showing up on the Titan planet. The adults try to lock them up, but the kids realize something is going on. It turns out there are some shapeshifting monsters on the planet that are masquerading as beings that they captured. A ragtag group of kids sneaks into preison to find real Titans and Olympians and escape to earth to prepare for a battle royale.