Yes, the grammar is fine. One thing to note, though: if you say "This is a teacher of mine," it is implied that you also have other teachers. You are saying that this is just one teacher ("a" teacher) among several. The same is true if you say "These are some teachers of mine." The wording implies that you also have some other teachers, not just these ones.

Its perfect !! Great job.

I think it's better to say : this is my teacher! Those are my teachers... Well it calked heavy english i think

As for me there is no problem.