The number one most important thing in modeling for art classes is to be in class and on time for each and every booking you make. A person can strike the most interesting poses and hold them perfectly, but if that person doesn't show up for classes or is habitually late, then that person will be regarded as unreliable and will not get many bookings. Modeling is a job (although I personally tend to think of it as something that I "get" to do rather than something I "have" to do to earn a paycheck), and every other job in the world expects one to be on that job and working at the appointed times.
Life drawing classrooms (or studios) can vary from school to school. There is a model's platform, and there are usually easels, drawing horses, or both, although occasionally a room will just have work tables. Most drawing teachers prefer their students to stand at easels so that they can occasionally back up and look at their work from a little bit of distance. It's a little more difficult to get students to stand up from a drawing horse to take that step back. The model's platform can either be up against one wall of the room with the easels/drawing horses arranged in a semi-circle or in the middle of the room with the easels positioned all the way around. I call this second scenario "drawing in the round." I find the semi-circle arrangement a little more advantageous for me as a model since everyone in the room will get a somewhat similar view of me. With the platform in the middle, some students are getting a back view of me. Back views can be good, but if I'm sitting in a chair, there's a chance that someone with one of those back views will get more chair than model. I tend to place a lot of value on my time for these students. One can draw a chair almost any time, but one's opportunities with a live nude model can be very limited.
Some studios have a private area blocked off for the model to change, etc. This could be a dedicated changing room, a closet, or just a corner of the room with a privacy partition. If there isn't a designated changing area in the studio, the model will sometimes change in a restroom and walk back to the studio in a robe, carrying his or her clothes. Personally, I hate having to change in a restroom and will only do so if there is no other option. Restrooms are for ... well, we all know what restrooms are for, and I would rather not have to change in one and risk my clothes falling on a less-than-clean floor, having people walk in on me to use the restroom, etc.
Once the drawing class starts, the instructor may or may not have a mini-lecture. I, as a model, am always ready to go at the beginning of class time regardless of this. More often than not, the drawing will begin with a series of short gesture poses. Whenever the instructor gives me the go ahead, I drop the robe quickly and confidently, especially if it's a class that I've never modeled for before. I want to appear to the students to be very much at ease and comfortable in modeling in the nude. I've read/heard from artists who thought that the model looked very uncomfortable on the platform and as a result, had trouble getting comfortable themselves while trying to draw. These beginning gesture poses can range in length from 5 seconds up to 3 minutes. Since these poses are so short, I tend to take a lot of action positions that I could never hold for longer periods of time. I do a lot of them with sports in mind, baseball, basketball, football. The discus thrower and the archer are personal favorites. And I'll also try to do some narrative poses, i.e. my hand shielding my eyes from an imaginary sun with my other hand pointing at something in the distance. Here are some quick drawings of me from 2011. You'll probably notice a couple of classic baseball poses, a batter at the plate (the artist added the bat) and a catcher in a crouch, ready to receive a pitch.
Once the gesture poses end, we'll go to a longer pose, usually ten or twenty minutes. Some classes will stick to twenty minute poses for the rest of the class (the typical drawing class is three hours long). Others will have progressively longer poses as the class goes on, even to the point of having a one-hour pose at the end. The instructor will typically give the model a bit of a break between every pose longer than twenty minutes and a break in the middle of the one-hour pose (getting back into the same pose after such a break can sometimes be a challenge, but over the years, I have gotten pretty good at it). And the entire class will take a ten or fifteen minute break about halfway through the three hours.
Some drawing classes (and almost all painting classes) will want one pose for the entire three hours (with breaks every 20 to 30 minutes, of course, although I sometimes go 45 minutes or longer between breaks, depending on the pose). These positions are typically seated or reclining poses with the props set up on the platform by the instructor. I find the reclining poses the easiest, as probably most models do. Seated poses can be problematic. Sitting can sometimes block blood vessels and/or nerves, and I will lose the feeling in my foot/leg. I've come out of some poses unable to stand up for several minutes as the blood flows back into whichever foot went to sleep. I've learned that trying to stand up immediately after such a pose will cause the blood to rush back into that foot way too quickly. The result is a sensation that feels like a thousand pinpricks in the bottom of the foot. If I turn on my other hip and keep my foot somewhat parallel to my pelvis, the blood will flow back at a slower pace, and that painful sensation can be avoided.
Standing poses can be tiring, but I seldom lose the feeling in any body parts during the pose. Some instructors are surprised when I tell them that I don't mind doing a three-hour standing pose, even to the point of not believing me. But I really don't mind doing them, especially if I can get a somewhat equal weight distribution between each foot.
My preferred type of class is one that starts off with a few gesture poses and then does 20 to 30 minutes poses the rest of the time. Those are easier on my body, and I'm not confined to one pose for the entire session. However, those three hour poses can result in some exqusite work by the artists in the class. The following is a piece from a three-hour pose of mine by Julon Pinkston in 2008, who at the time was a graduate student at the University of North Texas.