The decision to use a 504 Plan implies that the child is able to function in a regular classroom setting, but they may need accommodations in order to succeed. A 504 Plan is not part of Special Education. In public schools, there is something called a Least Restrictive Environment (LRE), which means we want kids to remain in the general education classroom unless they really can't function there, so in the public schools, teachers and administrators work hard to find ways to help children succeed in those regular classrooms so they have access to the same curricular content as everyone else. Lots of things can get in the way of academic success. 504 plans remove some of those barriers.
If a child has been on a 504 plan (as many children diagnosed with ADHD are), and the accommodations are not helping a child reach their academic goals, teachers or parents may revisit the question of Special Education, but in most cases, the 504 Plan is sufficient. This 504 plan is a document that will follow the student from year to year, classroom to classroom. It formalizes the use of accommodations and interventions that tend to work with this child. This is a good thing. It means it's not necessary to reinvent the wheel every year, and it will save your child from experiencing frustration with new teachers who may not pick up on what your child needs quickly at the beginning of the school year.
If you would like more details, I recommend this article on LD Online: Understanding the Differences Between IDEA and Section 504