You know, classes have a distinctly different feel to them of there are an odd number of students as opposed to an even number of students. I think this mostly boils down to the fact that if an odd number of students show up for class, the sensei has to pair up and work in with the class or else do some juggling of students. When an even number of students shows up, the sensei can stand to the side and look mysterious and inscrutable. Sensei gets much more exercise when an odd number of people show up.
But if you reduce this to minimal class sizes - like only one or two students showing up, some other interesting things show up...
- The koryu relationship - Back in the day, when much of this stuff was transmitted from one master to one student in private lessons, the sensei frequently played the role of uke (attacker). Makes sense... The sensei already knows the tori/nage part that the student really wants to learn, and the master can provide better ukemi. More precise, correct attacks by the master make for much better performance and learning from the student. This sort of thing was more common in the koryu (ancient) arts, and you still see this sort of arrangement in a lot of weapon arts - typically the attacker is the more experienced person in the relationship.
- The gendai relationship - But a different sort of relationship sprung up in what I might call the "classical period" of the gendai (new) budo (judo, aikido, karate-do) - that is, the first half of the 20th century. In this new relationship, the student was often expected to take ukemi for the master for an long time before he got to be the tori/nage. In this sort of class, the master would sling the student around and the student was expected to "steal the techniques." Students became adept at eidetic learning because they had to - there was no way the master was going to take falls while lecturing an inept student. The student might not even get to be uke until another, more junior student showed up.
- The western relationship - And then a third type of relationship emerged - perhaps we could call this the Western period, when the knowledge had been disseminated sufficiently that westerners (who were used to more egalitarian, democratic relationships) mastered the arts and got their own students. More often in this sort of relationship, the master and the student would be expected to alternate between the roles of tori and uke. I recall one of my instructors, who sort of started on the cusp of this third period, described getting abused by his instructor until he became skilled enough to put his foot down and demand more egalitarian treatment - as in the master takes as many falls as the student.