I wouldn't recommend anyone telling their geography teacher they "ma kota" considering that most Polish speaking people would understand it as you're telling them that your teacher is crazy.
What if someone really has a cat? How do we say it? Why is having a cat crazy?
I have no idea why. It's not like English "go nuts" is much better.
There is no other way to say that someone has a cat.
It depends on context, but could in some cases be left ambiguous on purpose.
Where does the sentence lead? to next sentence about cats, or to next sentence about a person being unreasonable/crazy.
In my experience many crazy people (especially crazy women) have cats, so it's a good expression.
I was very happy to read this comment, because I am reading a lot of Chmielewska now, see this expression pretty often and was wondering if it was obsolete (some idioms may be in and out within decades) while I have only just learned to recognize it )))
Frankly, while it's obvious to me what it means, I have the impression that I haven't heard it for ages.
having cats is very important for geography teachers