Oh no~~ I still have a school library book. And school is close for the summer.what should I do?

I still have one of my school library book, and they say that if we dun reture those book we will be block when we register for next year. I'm scared now ~ cuz I didn;t know i have that book until today. I tried to call the school ~ but no one answer. so please tell me wat to do. I'm enroll in skyline oakland ca, gonna be a junior next year. Wat should i do ??