Kendra is an 18 year old high school senior. She is an attractive and reasonably well endowed young lady. She seems pretty normal, but Kendra has a secret, a secret she learned on her 18th birthday: she can transform into something not quite human.
Fortunately, she is able to blend in with society because she can do something to control her transformation, to prevent herself from accidently transforming when it might expose her secret to the world. (Examples: takes a potion, performs a certain act or ritual, transforms herself voluntarily for a certain amount of time)
Unfortunately for Kendra, this morning she overslept and was in a hurry so she didn't do what she normally does. And now it's midway through the day at school...