New to Khan and need help!

Hi! I am a sped teacher and am hoping to use Khan to help guide my student's learning in math. I have a classroom set up and have my students in "early math review." I have just been having them start at the beginning and work their way through the course. Is this the correct way to use Khan academy? I plan to have them work through the course and when I see that they are having trouble with a particular item, I thought I would then assign it. I am just not sure how it works; once you select and start a course, is it common to just let them start from the beginning and work their way through? 

I am also wondering what the different levels show in regards to a certain skill(i.e. proficient, mastery, etc.)

Thank you!