ability to set repositories permissions per branch

I can choose if a repository is public or private. If I choose it to be private I can give fine grain read or write access to individuals or groups of people.

I would like to be able to be able to define a repository as private, but give public access to specific branches.

An example where this would be convenient: I lecture students in C++ at university. I make available some source code (master branch), and assign to the students some tasks to enhance it. The solution, which I share with the Students after the exam, is in a branch. I would like to be able to set the master branch as public and the solution branch as private.