Course Units have a checkbox saying: [check box] "User needs to complete current unit in order to access the next one"
However this is not restricting permission to the units for a student.
On the demo I set up here...
All the units should be restricted however as soon as you sign up any unit can be completed in any order.
(This should be quick to replicate by creating a new course adding 2 units with the checkbox ticked. )