Bookworm - I would not expect a patient to lay there in their bra and panties during an exam - I would expect them to have an exam gown on. And for your modesty the school should allow you to wear one also. If you are afraid that they would not have one to fit you purchase your own now and take it with you.
Nursing uniforms go up to 5X. And on some specialty sites you can get them up to 8X. It should not be a problem to get a uniform for school. If you go to clinical and you have to change into facility provided scrubs
some places only carry them up to a 3X so if you wear a larger size that may be a problem.
If that is going to be the case talk to your instructor in private. I have scrubs that belong to the school for students at clinical who wear XS, XXS, 4X, 5X. I also have a few scrub dresses for students who do not wear pants but want to go to the nursery or labor hall. These are kept locked in my locker on the unit, I give them out if I have to, and the school has an agreement with the facility to launder them and return them to me. Your instructor will work with you (I hope).
Good luck with school & your weight loss - of the two - weight loss was/is 100 times harder for me.