To grant access for a file or folder only for educators, for example, you have to right-click on the respective file or folder and then select „Get info”. This will open a box from where you can select „No” to the option „For parents”. Thus, only educator will be able to see that file.
To give preferential access to a file, right-click on the respective file, then select „Get info” and the push „Edit” button. Then another box will open from where you can select the person/s you give access to, but also modify the name and the description of the file.