When you create a document or even upload one to your Google Drive it is not shared but only available for you to see. So if you are going to set a link to a document in your classroom you will need to change the share properties. Here are the steps to take.
Then Click on the Advanced Link.
You will see that it says Private Only You Can See. So click on the word Change.
At this Point you can use what level of sharing you want to set.
1. Anyone can see the document.
2. Only those that you send the website link to
3. Only people with NSW Education credentials (ie signed into Google with education email account)
4. Only those that are signed in to the Education account and you have sent the link to.
5. Or only to people that you list to share with.
Click Save and then Click Done.