Relax. Not simply the usual advice about not stressing but really relax. If circumstances allow, do a few minutes of relaxation exercises before you start. You will (usually) be expected to hold a short series of poses ofr just a few minutes each. That rapid sketch time is for the artists to warm up. You then need to go into a static pose that you will be expected to hold for quite a long time. If your muscles are tense or you are stressed as you go into that you are more likely to cramp up. Ask the teacher to make chalk marks on the ground to show where your feet or whatever are. So you can resume the same pose after a break.
Learn to flex and relax your muscles without altering your position. Then you can ease the cramps a bit without changing position. Guardsmen are taught to tense and relax their calf muscles every so often as that reduces the chances of them fainting.
Have something to think about. Compose your shopping list or plan a holiday or dream up a short story.
If you have never been in a life class you may be surprised how formal it all is. You do not wander around starkers!. You walk to the podium in a robe. Your robe is brought to you when you break.... Etc. The artists will usually be very quiet and the whole atmosphere is like being in a library!
You will be surprised how quickly you find you have forgotten about being nude.
Enjoy it. I have known a few life models and the good ones all enjoyed doing it and were a bit exuberant. One chap in particular, delighted in dreaming up ever more odd or challenging poses. You may not get that choice as the teacher may well dictate what is wanted.