In the general channel for a classroom team there is a folder called, "Class Materials". This folder is setup automatically for all classroom Teams. It is great because you can put files there that students can download and view, but not edit. 

HOWEVER, it turns out that you can make any folder (or file) in any channel of any team read only for students.

Here's how

Short version:

The longer but video step by step version

Click here https://www.youtube.com/watch?v=BpKZOuEswEE