First of all, thank you for putting up the question. Concentrating in online class is difficult especially for someone who has been taking classes physically so far. Need to come out of the reluctant zone. I remember my experience when I first took classes online. What I thought then is, I will be able to cope with it as the time passes by.
Besides, keep your focus on learning and try to abstain from things which distracts your concentration while taking online classes.
I am sure you have the text books, read what was taught on a particular day. Everyday reading is a very strong habit for effective learning.
And please keep your will-power strong to learning no matter what !
I hope these tips will help your cause for concentrating..Good Luck