Having been on both 'ends' of a life drawing class..
If it's for life drawing/painting go right ahead. It's a good job and pays pretty well, although having to hold a pose absolutely still for a very long time can be difficult. Don't be self-conscious about your body. I've found part of the point of life drawing is that you draw a range of people with different body types. It's not like other forms of nude modelling where you won't be invited back if you don't have a rippling six pack.
I would be very wary of having anyone take photographs though, even for reference purposes, especially if you plan on later getting a job which might put you in the public eye. It's up to you though. Facial recognition software is going to be a big deal in a few years, and all it's going to take is for someone to put those pictures on the internet..
When you're doing it, don't make eye contact. Remember, they're focusing very intently on you and it's probably a bit awkward for them too depending on how long they've been life-drawing. Just try to put on a professional demeanour, and make sure you have something to think about because otherwise it will be agony (you really do have to stay absolutely still). Find something which requires thought (I was drawing up wargaming lists last time) and plan it in exacting detail in your head, as long as it takes your mind off the situation.
If you explain to whoever is running the class that it's your first time, they may well make some concessions for you, such as splitting the class up to allow for shorter poses. Of course, that depends on the person in question and their needs so don't expect it, but it's a possibility.