Are studies centered around one's own students regarded as publish-worthy scholarship within educational research?
Within educational research, is it regarded as acceptable practice to publish research which is only conducted on such conveniently available groups?


Sadly, yes. In fact, some of the most noted and most often quoted work on the psychology of learning was conducted by Jean Piaget on his own children.

However, there is a benefit to conducting such small studies of the type you describe. In the type of study you describe, where an instructor teaches two sections of the same course at the same institution to the same body of students that are otherwise identical except for whatever change in pedagogy the instructor desires, many confounding variables are inherently accounted for. Principal among these are the "instructor effect" and the "student-to-teacher ratio". Additionally, two sections at the same institution should be influenced similarly by demographic, socioeconomic, and academic effects on the student population.