Check your book bag, other rooms in the respective houses -- dining room, kitchen, living room, bathroom (it may have gotten stuck on a shelf, by mistake). Check with friends; perhaps you left it with one of them. Also, check cars -- parents' , yours, etc.
If you still cannot find the book, tell your school librarian. Perhaps you can have an extension on the loan period so you can find the book. At a last resort, you can offer to purchase a replacement copy.
Hope this helps. Good luck with your search!