2. I've got a certificate
The second type of writing teacher has hung up their shingle because they’ve completed a degree or diploma in writing; for example, a master’s degree in creative writing. They might, of course, be a wonderful teacher with lots of helpful information to impart. But let me explain the potential problem by way of another example.
I once saw a brand-new history book selling for $5 in a "remainders" bin. It was written as part of a post-graduate degree and had won an award. It looked a bit quirky, so I bought it. Within a couple of chapters, the quirkiness had become irritating. Within a couple more, it was unbearable. I soon booted it into my own "remainders" bin.
It reminded me that if you mention to a literary agent or publisher that a manuscript was written as part of a post-graduate degree, they try not to roll their eyes. They sigh and murmur that most manuscripts produced in these types of degrees are unpublishable.
Why? Because what university lecturers are seeking and what publishers are seeking are not necessarily the same thing.
Publishers are looking for books that the average reader will pay to read. So what motivates readers to hand over their hard-earned money? An interesting subject that is well-written and readable.
By contrast, university lecturers usually don’t care about the reading tastes of the general public. In fact, some would consider "the masses” a bit low-brow. What university lecturers are looking for is something that interests them within the guidelines of the degree they’re teaching. Thus, thousands of writing students graduate each year — many with "high distinctions" —but they soon discover that there's little if any publishing interest.
Some are successful but the numbers are small. Some keep trying … and trying. Some self-publish their manuscripts. And some hang up their shingle as a writing teacher, and teach other aspiring writers the same skills they were taught.
Some prove to be better teachers than writers. However, of the writing teachers I've encountered who are not mainstream-published authors, the best have spent years in the industry as editors or literary agents.
Thus, just because a writing teacher has done “a course”, it doesn’t mean that they can write the type of prose that the general public wants to read. Nor does it make them a good teacher in terms of helping others craft the type of prose that readers want to read. So keep this in mind as well.