If there are seven Atrians at the high school, why do we only meet four?

Seven is a fairly small number. Why did they only introduce four of the Atrians? Why not all seven? And if they were only going to introduce the four, then why not just say that there are four participating in the integration program?

Kind of annoyed me. Would have liked to get to know the others. It would have been one thing is the number was larger... but three extra characters (even if only a couple of lines in episodes here and there) would not have been to much to ask for...