I cannot believe the president of the hospital emailed your NM over the 'sweatpants' incident. I just can't believe that is true. So either: 1. (as someone already mentioned) if the president of the hospital is wasting time over such trivial incidents, well then, you may not want to work for that hospital; or 2. (as Ruby mentioned) there is a whole lot more to this than just that you were wearing sweats. There basically has to be more to it if so many people are willing to make such a big deal of something that seems like nothing.
Also, I wanted to echo what Ruby said. Every time you show up at your place of work for a work related function, dress professionally (by the way, the fact you didn't wear make up is totally inconsequential one way or the other), whether it be your assigned shift, a meeting, a class, an inservice, etc. Unless the flyer for the class, etc. specially says 'staff may wear jeans and a t-shirt to this class,' dress professionally--even your work scrubs
are appropriate for certain situations, such as a CPR class. Also, never ever ever (never!!) wear anything that is controversial or even in the least bit inappropriate, such as a low cut top, a shirt with a vulgar saying on it, etc. You are judged every time you are there. Every time HR or you manager sees you, they are evaluating you. This is especially important if you want to advance your career by becoming an RN at this hospital.
This incident may seem unfair, but, as future professional nurse, you had to know better. Sorry if this sounds harsh, but it's reality.