http://classroom.booksource.com/ on the web and the Classroom Organizer app https://itunes.apple.com/us/app/classroom-organizer/id476668797?mt=8. This site and app revolutionized my classroom library, how kids check out books, how they return them, and gave me a way to make them accountable.
Once you set up your account online and download the app it is all so simple. You simply scan the ISBN number on the back of the book with your smart phone or you can enter it in on the web, and it will catalogue the book for you. You can put a location where the book should be found and returned to. You can even set it up so that when students return books they give a summary and rating! Kids can check out and return books by using the app on an iPad, iPhone or a computer. It really makes your classroom library like a real library and they seem to take it more seriously. After a year of using the Classroom Organizer/Classroom Booksource I only had 2 books not returned, but over 500 books circulated. Try it out for yourself and enjoy!