In what context could this be said? Why is it unfortunate that my teachers are not the worst ones? The worst ones could be better?

> In what context could this be said?

For example:

Yes, my teachers are pretty bad. But, unfortunately, there are many teachers in our schools that are much worse. With the educational system like that our country is truly fucked.

@thanks deniko, but the subject sentence is not even included in your example.
I'm still not convinced this sentence makes sense. Perhaps, in the form "Unfortunately, my teachers are not EVEN the worst ones." But without "even" I find it strange.

Well, I just provided a possible context in which this sentence might be uttered - discussing the state of affairs in the educational system.

For all that it matters, it doesn't sound strange to me, given the context.

We can absolutely use this exact sentence without adding "even" in such a context, I believe.

Another possible context - talking about my school in particular.

First, I say our school is really terrible. To prove my point, I go on a rant about my teachers. There is a lot to talk about, so it takes me some time. Then I realize I need to return to the point:

"Unfortunately, my teachers are not the worst ones. Mike's PT teacher literally yells at the children! And Julia's maths teacher came drunk to the class the day before yesterday. "

Thanks for the explanations, I steel feel it's strange, but that's not a problem.