What if some students finish a course?

If I have been implementing a CSF course (e.g., Course D) for a while and students are working through in a semi-self-paced fashion, some may finish the course and be ready for the next one (e.g. Course E). Is it possible to have different students in the same section in different courses?

Glad to hear that you have been using CS Fundamentals course D with your students! Don’t forget to include the unplugged lessons to support their learning.

To answer your original question, you are not able to assign more than one course at a time to a section. However, students can access and progress through any courses. You can learn more here: https://support.code.org/hc/en-us/articles/360003446052-How-to-change-course-or-unit-assignments

Thank you. I didn’t know that the course assigned was just the first course shown.

First of all congratulations to your students.
Actually, IN CSF course, you are not able to take more than one course at once.
In CSF course One course provides at a time. Now you can take practice exercise to command on that section on which you working.
Hope this will help you