Not closely associated with the channels - which are probably sections of your curriculum or weeks - depending on how you organize your channels
Actually in an Educational setting, one usually wants all files in the files sections of the various channels to be Read Only - or am I wrong? If you want any learner to submit an edited version of a document, you will usually request it via an assignment submission. At least that is my take on it.
So here is another way in which you can make your files read only in the various channels right now and accessible on all platforms - Android included (actually it is alluded to here, but they make it sound much more complicated than what it is):
Open the files tab in any channel of your team
Choose open in SharePoint (if the option is not visible - first click on the ellipsis ...)
You will land on a page showing the contents of that folder with Documents>ChannelName at the top. Now click on Documents (either there or on the left). That will now show you all the folders of all your channels.
You can now choose them one by one and set them to be Read Only.
Select the folder (don't go into the folder)
Select the option on the right that says Manage Access
Below that you will see the group name "YourTeam Members". Click on the drop down button next to it and set it to Can View - that's it!
It may sound like a long process, but once you've done it once or twice, it is so simple you'll be able to do it in your sleep. Forget about Class Materials!
P.s. If you only want sub-folders of your channel folders to be Read Only (and not the whole folder), I'm sure you can work that out ;)