This is wonderful you are implementing a 504 for your child, it was the best thing I have ever done for my daughter!
The things that are included in her 504 are study time right before a test and if she feels she is not ready to test at that time acommodations are made for testing later, peer tutoring....which has made her life so much easier having antoher student reteach her what she has learned that day or week, extra help from the teachers when needed after school and extra time to make up work she has missed due to absences.
A 504 can be tailored to your child's needs, there is a family here from NJ that has a 504 for each of their 2 kids with lyme and they did not get a hard time, I actualy believe things got easier because before the 504 teachers and administrators were getting frustrated with the kids and thought the were just ADD.
By law the school should not give you a hrad time, you will meet with a guidance counselor and a school psycologist who is there for the best interest of your child. Good luck....you will be happy you did this! Peace!