The Olympian gods thought their kids needed protection but also to learn so they turned camp halfblood into an academy. They have dorms and classrooms, but something big has been brooming so now most of the kids are in class learning battle tacktics and battle skills.
The gods needed a special group of demigods to protect the academy and their kids. So the gods chose twenty-four kids two from each god to form a group to do just that. (When we get gods and they claim kids the first two kids for each god will get to be in the group.) The group is not a secret but the other demigod fear them thinking they have special powers, dark powers.