Tynker, by default, assigns every lesson in a course. You don't have to accept this if you don't want that. Simply go to your Class--->Lessons tab---Lessons Button on that course.
Here you can go to the bottom of the page and click the three dots to unassign any lessons. Those lessons will now go to the Unassigned Lessons tab (which will appear)- when you want to re-assign them.