The Visitor: Animorphs, Book 2

Rachel is still reeling from the news that the Earth is secretly under attack by parasitic aliens known as the Yeerks. And that she and her friends - five kids who, purely by chance, stumbled onto a downed spacecraft and were given the power to morph into any animal they touch - are the planet's only defense. But Rachel's always been something of a daredevil. So when it's suggested that they infiltrate the home of their assistant principal, Mr. Chapman, who also happens to be a human host of the Yeerks, she volunteers. But what she finds inside may be more than even she can handle.