Princess Candy Series by Michael Dahl, Scott Nickel

At Halo McNightly's school, the swimmers are disappearing from the boys' swim team. No one can figure it out, until Halo stars investigating. A deadly mermaid lives in a cave connected to the school swimming pool. Halo will have to use her special candy to turn into various underwater fauna, such as a hammerhead shark or an octopus, to defeat the hungry girl-fish!

The teacher, Mr. Slink, assigns an environmental project, and Halo Nightly pairs up with Flora, the school's resident treehugger. Their team has the assignment in the bag (reusable, of course). But when the evil Doozie Hiss ruins their chances for a good grade, Flora changes from happy hippie to eco-terror. With her superpowered candies, Halo faces off against the Green Queen of Mean!

With the evil Echo around, Halo Nightly faces her strongest villain yet herself. Echo is a sneaky shape shifter that morphs into Halo and causes her all sorts of problems. When Echo steals Halo's candy, Halo is forced to battle someone who is every bit as powerful as she is.