First of all, don't worry too much! If it's a regular class, then the person running it should be adept at making their models comfortable. They should provide a robe, adjust the heat to your liking, make sure you're in a comfortable position that's easy to hold, and ensure a respectful attitude in class.
That said! Bring your own robe/huge jumper if you prefer to. Ask for the temperature to be adjusted to suit you. Speak up if the pose feels too exposing. Speak up when you're in physical discomfort and need a break.
Don't get fancy with the pose unless you're super fit and strong and bendy. Even the clichéd 'lying with head propped up on hand' is incredibly hard to hold for a first-timer. There's a reason so many nudes are reclining!
If they want complicated action poses they'll recruit at the climbing wall (my first few times) or invite the local farmgirl (my second, regular gig) :D
When it's time for a break, don't move immediately. Take a few seconds to think about how your body is arranged - it'll help you get back into position afterwards. Then move out of pose slowly, in case you've cramped up.
Smartphones and photos weren't a thing in my day, but I'd hope there are clear boundaries preventing their use.
Anything feels wrong to you, at any time, speak up! Working from a naked body is precious, so your comfort, on all levels, is crucial. Without you, it's a bleeping bowl of fruit again. Remember that!
I've assumed an art class, but it all applies to a solo session too.
For me, there is nothing quite like the quality of seeing, of attention, that you receive as a life model. Everyone is totally intent on some aspect of you, as they observe it, in minute detail; and yet it is not at all personal. The artists are focused on lines and shapes and colours, trying to capture what they see, each so differently, expressed in a multitude of ways. In the right setting, it can be a very special experience.
Edit: ps. I was always paid!