The most appropriate way to do this for a class that should understand Git well is for you to, as others have said, is to update your repo, and make them fetch and either merge, rebase, or cherry-pick the commit(s) in. Cherry-picking is probably the best solution, since merging and rebasing aren't really designed for a workflow like this. (You could probably provide them with a script to do so.)
The easiest way for them is for you to either:
- open PRs on every student's repo with the given commit, which you could script with Hub, or
- push the commits to every student's repo.
In both cases the students will have to pull down your commits; there's really no way to get around that.
I wonder if this is a common enough use case where the Classroom interface should support opening PRs into every student's repo - I've ran into this two or three times this semester.