Chookie, sounds like a tricky situation, you have my sympathies.

I was curious about the fact parents were there, because I think that
always creates a difficult environment for the teacher and the parent,
the parent will hold of stepping in, thinking the teacher is in charge
and the teacher isn't as strong as they might be if the parent wasn't
there, though it does sound like the behaviour of this child adds up to
more than that.

We had a similar situation with DD's gymnastics last year, there was one
child who pretty much took all the teachers time to get him to behave
and as a consequence she had to let him get away with some of the less
bad stuff, like sitting safely, but somewhere he wasn't supposed to sit,
we then had a problem, because DD would see him sitting somewhere she
quite fancied sitting too and go an join him! The difference was, it was
clear that the teacher and the parent were well aware that it was a
troublesome child and the class only had a couple of months left to run,
so though I moaned to DH everytime, we had also made the decision that
we'd just not sign DD up for the same class next session rather than
complain to someone at that time.

In the end I think that's the choice you have to make, it's not fair
that your DS should have his class disrupted like that, but your choice
is to not let that bother you and focus on how much he does get out of
it, or approach the teacher (or someone who is above them if they
exist). You could suggest to the teacher that parents are encouraged not
to watch if you think that would help them and if not, you can always
remove yourself to avoid stressing yourself out!

Cheers
Anne