i don't know any 7th or 8th graders, but maybe start with a list, and winnow it down:
1. grub -- or other -- bootloaders.
2. filesystems -- "disk" partitioning.
3. some different distros . . .
4. ngnix
5. a database manager ?
6. an overview of development tools:
a. the gnu compiler family
b. git and revision control systems
c. languages?
7. html, website construction

probably can't cover all of this in a semester, you'll need to pick and choose