In the US, police and firefighters are usually unionized. There are limits on their ability to strike. I believe that those limits might be in place for teachers as well.

The military has incredibly strict rules for advancement and compensation that are very union-like. They have defined ways to make a grievance. They get pretty good benefits and pension. I'm not sure that they'd choose to unionize or what exactly it would get them.

There is, of course, the job itself, which they have zero control over, and would probably not be able to negotiate even if they had a union.