We have a 504 in place and I think all kids with Crohn's should have one.
Example: my son's math teacher was going over material and my son went to the bathroom because he wasn't feeling well. He came back 20 minutes later to find the class taking a test on the material the teacher just went over. He ended up getting an F on the test because he missed the review AND didn't have enough time to finish since he was in the bathroom with the big D. He was absolutely devastated since he's a self proclaimed "math geek" and straight A student. As a parent, I really didn't care about the F itself, but what really bothered me was the emotional toll it took on my son. These kiddos go through so much already with this disease, they shouldn't have to worry about the other stuff that can be so easily avoided. A 504 helps with that.
We also had one particular teacher who wouldn't let him use the bathroom when he needed to. She thought he was going too often. My son took it upon himself to just leave and go use the bathroom in the nurse's office. He felt awful about disobeying the teacher though. Again, a 504 helps.
And I agree with all who said it's very important to get the 504 in place BEFORE your child needs it or gets sick. The trouble we had was that our school didn't see Crohn's as a disability. How can going to the bathroom 15 times a day not be a disability???? Eventually, they did put together a 504 for him, but I was given very little input. They did everything and parent involvement was a "courtesy". It basically says his disability is "digestive" and the impact on his life is "self care and eating". He's allowed unlimited bathroom privileges (that makes me laugh - privileges! as if it's a reward!), and extra time for assignments when he's not feeling well. They denied my request to provide extra text books for home during flare ups and there is no mention of stop clock testing. I need to get that changed.
It's also my understanding that a 504 carries through college, but one cannot be created while in college? Not sure on that one. Think final exams, SAT, etc.