Well, at the beginning level the classes are pretty simple. The beginning student is usually taught only rumba, cha cha, and jive. Group class is usually an hour to an hour and a half in length. The students would come in and usually rumba then cha cha would be taught. Because most of the group classes are usually 2:1 (or even 3:1) girls to guys, a lot of solo excerise is performed. A typical class would start with rumba music and everyone doing the basic facing the mirror. The technique, timing, and posture would be emphasized. After such a warmup, then the students might be asked to separate, girls one side and guys the other, to go over the class routine. The steps depend on how long the students have been dancing, but it would usually be basic, under arm turn, 3 new yorkers, under arm turn, 3 hand to hand, spot turn (or under arm turn), and back to the beginning. Then a fan might be added to a baseball bat before the new yorkers. As each step is added to the routine they are drilled over and over as to what the technique needs to be. Most of this is done apart then they do it together with the 1st partner then 2nd (and 3rd) partner(s). After some time of this they might move onto cha-cha (which would have an almost identical routine) or they might stay with rumba, working on more technique with excerises such as rumba walks, side steps, working on the new yorker/hand to hand action, etc.
Again, what exactly is done in class is not as important as to how it is done. The teachers demonstrate a lot as well as go around and fix. The teachers will call out people who are not doing it right in front of the other students as well as call upon students to demonstrate in class. This sort of Socratic Method is very effective in making students readily aware of what they are doing correctly and what they aren't. Also, repetition helps develop the dancer faster. Many times, group classes keep going in an attempt to keep everyone interested. This is not so in most of these classes. The teacher might do just rumba walks all class or just the basic, and an entire month of class might not go far beyond a couple of steps. This is done to ensure that the students are developing the proper muscle memory. These actions need to become second nature and what better place to get your muscles conditioned than in class?
I hope my outline helps you understand what goes on in these studios. I must say it was hard to recall a single class because they all kind of blur together. If you want to know anything else, just ask.