Personally, I think it is essential. I am guessing that as a teacher, you should be a role model to the children. Correct?

Consider the following scenario:

You are the member of a forum and children from a school know who is hidden behind the identity of a name and start mimicking their teacher's grammar, then I would say that the teacher is setting the child up to fail.

I have heard cases of children identifying someone on a forum and acting in the same way as that person. The 'adult' was usually quite insulting and had no regard or consideration for many people and the children did copy their attitude and then it led to other things. Perhaps the 'adult' should have considered their own attitude in the first place.

So yes, good grammar should be used and taught by a good teacher.