I've figured it out. I should have used classgroups::pk_classgroup id instead of meetings::fk_classgroup id in my meetings layout to summon up the list of students.
My plan is to choose a class group from a list of class groups at the beginning of each class period. Can anybody suggest a way that this could be automated so that depending on the time of day and day of the week that Filemaker chooses the class group for me. Or is this just over complicating things. I guess I could match classgroup id to class group id AND teacher id to teacher id AND time to time. But sometimes the class can start early or late. Is it possible to match using an approximate time?