The qualifications for teachers (and many other professions) has changed over the years, all over the country. Little doubt that qualifications during those years was very different than for today.

Because you've found your relatives to be Free People Of Color, it's likely that they were also literate. My guess is that if they could read, write and perform (at least) basic arithmetic tasks, their skills would be suitable for teaching in some of the K-12 grades.