The simplest way to record grades from students who take your course is to ask them to email you their course certificate, after they complete the course.
They can also send you a screenshot of their course results page, after they click the button to complete the course.
Now, we actually have a much more sophisticated way of getting grades, that was designed to allow for anonymized analysis of grades across many, many students. This feature was designed to allow Leanpub courses to scale to the level of the most popular MOOCs.
You can find these features on the "Quiz Responses" page for your course here:
...making sure to replace YOUR_COURSE with your course's unique web address.
To manage the grades of students in your classroom, you can ask them to share the unique Invoice ID they will get in a receipt via email, after they purchase the course (this includes free purchases of the course, including coupon purchases).
Then, you can match the Invoice ID to the data we provide, when you click the "Generate CSV" button on the "Quiz Responses" page.
For Leanpub authors familiar with using APIs, at the bottom of the "Quiz Responses" page, you will also find instructions for how to use our API.