Getting Out Of Swimming?

I took swimming last year in PE (I'm in high school by the way), and it really sucked. People get out of the unit for various reasons like medical or religious and are assigned a study hall. I could really use the study hall, even if it's just for a few weeks but my parents don't seem willing to get me out of it. I want them to write a letter saying they aren't comfortable with me using the swimming pool this year because there has been a case of MRSA (staph infection) at our school. Also I know a girl who got ringworm from school swimming, and a few years ago the swim team lost all their hair. I've had earaches from swimming before but not for a long time. Do you think that the school would accept not wanting to swim because of the MRSA going around? And any tips for convincing my parents to write me a letter?