I used to be an education professor, so I have some idea what your audience is like. I think they’ll be OK and that you don’t need to worry.
There is no disabling markdown, but as you know Markdown is designed to be pretty much what you’d type if you didn’t have a GUI. Your users can just type stuff and leave a blank line between paragraphs. The biggest problem you’re likely to have is those people who still think that you need to type five spaces at the beginning of a paragraph.
You could contrive to use CSS to hide the toolbar in the composer, but I think that’ll make things worse.