Lesson are opened with time. This is similar to if you went to school for 1 day a week. You would attend the lesson and the following week you'll have access to it.
For the online class, once a lesson is unlocked, you will have access to it and all the past lessons.
Make sure to click the Green 'Complete Lesson' Button before continuing onto the next lesson and to click 'Mark Unit Complete' at the end of the unit.