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