Use case: 

I'm a student that has an AP exam coming up. I want to maximize my score by studying my weakest portions of the course

Problem: 

In order to find and practice my weakest skills, I need to look within each unit manually 

Proposed solution:

Introduce a button adjacent to the "Course Summary" text titled something similar to "Mastery List". When clicked, a box opens up in the same shape as the course challenge box with each skill listed on the left ordered by mastery (least to greatest), identical in formatting to the list of skills 'leveled up' upon completion of the course challenge. The right portion of the box would import the four-question quizzes straight from each lesson. 

Alternatively, 

When the take course challenge/take course challenge again button is clicked, introduce a button on the left side of the white space that includes the let's start button titled something like "Custom challenge" that allows students to pick which skills will be tested. If this feature gains popularity, perhaps include a checkbox that uses the questions within each skill that the student gets wrong the most, and/or timed runs for AP prep, and/or custom question quanitites.