One of the hard things to get past, in teaching ESL, is the oft-heard objection from students that "my teacher in my country taught me that ___ is always correct."
If you're the current teacher and you're sure that you're correct, you still may feel weird telling a student that the past teacher gave them bad info. But consider the following:
--First off, you're likely to never meet that teacher, so why feel bad about the contradiction?
--The original teacher may not have spoken English all that well. Or maybe the past teacher spoke English well but was just teaching ESL as a way to get enough money to live in a foreign country for a couple of years.
--The original teacher might have been using books that had mistakes.
--The teacher may not have had a very nuanced understanding of the ways that the language is actually used in an English-language environment.
--Any teacher who uses "always" when talking about pretty much anything in English is pretty much begging to be contradicted anyway.
One of my more recent instances of having to contradict Teachers of Years Past came when an instructor asked me for clarification on which pronouns we should use with animals. The (foreign-born) teacher and her students had all been taught to use "it" instead of "he or she" with all animals.
Here's the thing. I have a dog named Lucy. She is a wonderful dog who gets walked frequently around my town and when I walk her, I often get into conversations about her. After I tell people her name, if they say "he" or "it" I will always correct them. And the thing is, I hardly ever correct people in conversations. But with Lucy, it would be a little bit like if I called your son "she" or referred to him using the word "her."
Basically, if an animal has a name and you've been introduced, it's expected that you use the appropriate pronouns (and I don't care what your past teachers said on this subject). This holds true for pets of all sorts (people might give you a pass on goldfish and guppies, but that's about it) and the rule also applies to a farm or zoo animal with a name. For instance, when I was in Australia, I had the opportunity to hold a koala named Charlie. From then on, I've always referred to Charlie as "he" (even though before I met him, I might have said something like "I wonder how long I'll get to hold it").