I could see how that eye should be darker overall, but I also think that losing contrast in that area, even if theoretically right can make faces look off very easily. So I think I'm in favour of seeing both eyes clearly, but it it were possible to make the eye a little bit darker I might go for that?

Really, this is a great portrait though! No matter whether it's techincally perfect, you can see the guy looking out at you. It's SAYING something and that's worth much more than the techinical stuff.

I'd maybe agree with those who said that the teacher might just be telling you stuff to make you EVEN better and technique can help you do what you want to do.