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.