I'm not sure about OneNote, but you can go a long way (even export PowerPoint and Excel files) with both LibreOffice (which is free for you and your students, forever) and with Apple's iWork, which is very expensive, but comes free with a beautiful computer. If you already own a computer beautiful enough, it's quite cheap.

Note: actually, iWork doesn't come with every new Mac, but I felt the joke irresistible.