Ellos son estudiantes promedio.

why is it not promedios?

I think the word average is quite unique, in the sense that when you say “they are average students” you don’t mean that each and every one of them is perfectly average. You mean instead that the collection of students together form a group of average quality, and so the word promedio isn’t acting as an adjective for estudiantes, but the unspoken collective group of them. For a more extreme example: son mujeres promedio is perfectly correct, where again promedio does not care about gender or plurality.