How to make a private pull request?

What I need

How could I make a pull request only visible to repo maintainer??

What I tried

I saw this article: How to make Pull Requests Private?

I have a private repo. All the students have read access to the repository.
I tested, still students with read access could see other’s pull request.

I tried forking the original repo
created a pull request: forked private repo -> original private repo
still, other students with read access could see the pull request.