Teaching Revision (Change) Control Systems In High School CS Classes
I agree with the CSTA Standards that revision control is an important skill by upper high school.  My school has a GitHub account and it is good for both revision practice and for using Agile boards for task management, which is an important methodology for students to know as they move to college.  I think there is value in either GUI or Command Line knowledge.  

Here are some resources about Agile in Education:
https://www.agileineducation.org/examples.html

I like this resource for revision control:
https://betterexplained.com/articles/a-visual-guide-to-version-control/

The main ideas are backup/restore, synchronize (sharing resources with multiple people), tracking changes, sandboxes, branching, and merging.