The first paragraph of any security course will ask you to begin with a) the value of your secrets b) who your adversary is.
You started by saying you want to secure "test grades, exam questions, ..., students personal info" from unauthorized users. Then moved on to hiding your users who "learn about democracy, human right, multi-party politic and freedom of speech" from the state. The two things are as different as a mouse and and an elephant. So please decide which one before you continue!
P.S. In an enquiry a couple of months ago https://moodle.org/mod/forum/discuss.php?d=358202 you talked of "the teaching staff involve in a moodle site are keen to see their online discussions on sensitive content (e.g. developing questions for the forthcoming exams, students marks and grades". How was the progress?
Security and privacy
https & sensitive info
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