I just added a collection, and I didn’t realize that there were around 150 lessons in the collection. I poked through the lessons and they are way too easy. I don’t really have plans to delete all 150 lessons individually and I can’t figure out how to mass delete or delete by collection. Having 150 unwanted lessons makes it harder to find the five that I want. The only thing I can think of to do is skip to the next level (which is too hard) and then filter for that level.
There isn’t a mass delete option.
You can, however, go to the My Lessons page and select List View (near the top left). Then you can sort by collection and click the tick box on the left to archive the lessons or the trash can on the right to delete them.
There are no limits on the number of lessons you can take from the Library, so if you prefer not to spend the time to delete/archive them, then there is also the option of just leaving them all there. Simply open the ones you want again to move them back to the top of the list in the Expanded view.