Yes. If your school has already bought ebooks from Overdrive or 3M Cloudlibrary, you can still add new titles from other publishers. You will just need to email the new ebook to students. When they click on the ebook, they will have a choice of opening it in Overdrive, Google Play Books, or any other ereading app on their Android tablets. If they are using Amazon tablets, send them a mobi file which will appear in the Attachments folder after they save it, and then load instantly when they click on it.
If your students plan to read the new ebook on their desktop computers, or their laptops or a computer in your library, they will need to install the Readium app for the Chrome browser, and then add the ebook to the library in Readium. See more about Readium here: http://ebooksforstudents.org/reading-ebooks-schools-without-tablets/.