We are in FCPS and you do NOT have to show an educational impact, in the sense that your child has to be failing. You DO have to show that there is an impact on their ability to "access the curriculum". That can mean "failing", but it can also mean needs constant re-direction to stay on task, reminders, social difficulties with students and/or teachers, emotional reactions outside the norm, etc.
A PP is correct in that it is generally easier to get a 504 than an IEP. A 504 just gives accommodations, which are generally of no cost to the school. An IEP means the student needs special education services. That can mean anything from modified curriculum, special placement in a class with spec ed push in services, pull out services with a therapist or teacher, etc. These are generally things that cost money, so schools will have a higher barrier to entry for these services.
A 504 plan in unenforceable. By this I mean, if a teacher wants to ignore any part of the accommodations in your dc's 504, you have no recourse.
This is not true. Schools are required under IDEA to comply with students 504 plans. I have found that a well placed e-mail to a counselor and if necessary the principal can get things remedied. Also, if the principal is not responsive there are higher level resources to appeal to.
But like the counselor above said, I have never found it necessary to be adversarial. I have a middle schooler in the AAP program who has an IEP for ADHD and anxiety. She gets special classroom placement for one subject (a team taught class with a spec ed teacher in the subject she struggles most with organization and social interaction) and a daily resource class with a special ed teacher for direct services. It was actually the school's idea to place her on an IEP. Before this she had a 504. She did have some teachers who were unwilling to follow her accommodations, but I have found the school to be responsive when I brought this to their attention.
My other child also has ADHD, more severe than my other child, but so far is doing OK with informal accommodations. I do plan to ask for a 504 for her this year because these accommodations will need to be in place as homework gets more burdensome in upper grades. Her teacher this year has a long list of accommodations that she provides for her, just because she's wonderful and gets it, but I know that we will not always be so fortunate with teacher placement.