When you are ready to share a file (or a folder, it works the same way), right click on the row of the file or folder, go down to share and choose share from the sharing options.
Check the permissions for sharing. For school files and folders I keep them as "Private - only the people listed below can access".
At the bottom where it says "Add People" add the email addresses of people you want to have access. If they are district staff or students, once you begin typing their name, the email address will show up for you to choose.
There is a checkbox if you want to email the person/people that you are sharing with. I have students uncheck this box so I don't get 100's of emails. Then just click "Share & Save".
The shared documents will show up in the other person's Shared Folder. You have the choice in how others interact with the file or folder. They can be set to View Only, Edit, or Comment Only.
One note - when you share a folder, any document placed or created in that folder will also be shared the same way.
Check it out and try it. It's awesome!