Here are some on the Git + GitHub side:
Number 1: If you use GitHub Classroom you can take Assignments with starter code and distribute them with one link. You can post it to your LMS or email it out to them.
While students will need to create their own accounts, I recommend collecting their usernames via a google form before class started.
Number 2: Classroom gives you a direct link to access student repos.
Number 3: Classroom allows you to keep repos private. Here's the README with that info:
Number 5: I don't know of a particularly good tutorial for high school students, perhaps @gcschmit has come across one?
I found these two, which seem straightforward:
Numbers 4 & 6 might be best suited for the Eclipse forums....let us know what you find!