OneNote has been one of the best kept secrets of Microsoft Office and a killer tool for students and teachers in the classroom. The OneNote product team has just released an update to the OneNote Classroom Creator that makes it extremely easy for teachers to setup shared notebooks online for true collaboration.
More details here: http://blogs.office.com/2014/10/07/introducing-onenote-class-notebooks-flexible-digital-framework-teaching-learning/
And here: http://www.onenoteforteachers.com/en-US
-Carl