CK12.org is what I mostly use to add lessons to edmodo.com. CK12.org has separate math and science apps that can be installed in groups of students that you are teaching. The lesson can be chosen as assignments or quizzes and give the students instant feedback. I use CK12.org assignments for homework and test prep. Having the assignment online means the student can’t loose the assignment.
I enjoy using CK12.org because:
*it is free for everyone to use
*it grades it’s self
*there are hundreds of lessons created
*it works easily with edmodo.com
CK12.org is more then just math and science apps, there are many e-books and other resources available on their web sight. As a high school math teacher I am using the CK12.org app on edmodo.com and not much else. I think for teacher of other subjects it is worth a look.
I could provide a half of PD on CK12.org. There is something for every subject and plenty of resources for math and science.