Only by understanding the origins and purposes of these two laws will schools and families be able to best apply each of them in the appropriate circumstances. Let's break these differences and similarities down to compare and contrast.
The IDEA is an educational funding law. It requires that each state that receives federal funds (and that is ALL of them) provide a free, appropriate public education (universally referred to as FAPE) to all K-12 students who meet the definition of disability in one of 13 areas. Among the most common areas are Specific Learning Disability, Speech and Language Impairment and Other Health Impaired (a bit of a catch-all, which often includes ADHD). The IDEA applies to all students who have been determined to have one of these disabilities, whether they attend a public or private school, although the funds available to students in private schools come from a different funding stream and can sometimes be more limited than those available to public school students.
Section 504 is a civil rights law. It is often described as a law that seeks to "level the playing field" by providing students with disabilities the same access and opportunities as students who do not have disabilities. It only applies to schools and school systems that receive federal funding, so it does not apply to most private schools (unless they have some sort of federal funding, such as for a lunch or enrichment program). Its definition of what constitutes a disability is far broader than the IDEA, and is defined generally as a physical or mental impairment that substantially limits a major life activity. There is no limiting definition of what kind of impairment "substantially limits a major life activity," but Section 504 notes that this definition includes (but is not limited to): learning, concentrating, thinking, reading, hearing, communicating, seeing, and working. So, students who have a defined disability under the IDEA will also fit the definition of a disability under Section 504.
Why, then, do we say that these are not interchangeable? A good explanation of the differences between these two laws can be found in a document from the U.S. Department of Education:
First of all, the IDEA provides very specific procedures for how it is to be applied. Details of evaluations, meetings to develop the IEP, and remedies if there are problems with any aspect of the IEP procedure are set out in the IDEA and its regulations with great precision. There can be some differences from state to state in how these work in practice, but every state must provide at least what is set forth in the IDEA. One of many aspects of the IDEA is that parents are a required part of the team that creates the IEP.
In addition, the IDEA provides a rich array of services, including special education and related services. Special education is defined under the IDEA as specially designed instruction, at no cost to parents, to meet the unique needs of a child with a disability and related services (things like speech, occupational, and physical therapy) are defined as supportive services that are required to assist a child with a disability to benefit from special education.
Under Section 504, school districts are required to develop and implement a system of procedural safeguards to address FAPE concerns specifically, such as the identification, evaluation, and educational placement of students with disabilities. Procedural safeguards include notice; an opportunity for records review by parents or guardians; an impartial due process hearing, with an opportunity for participation by the student’s parents or guardian and representation by counsel; and a review procedure.
Section 504 does not include parents as part of the team that initially creates the 504 Plan, only after the fact if a hearing is required because of a disagreement. And, while some states follow the procedures in the IDEA and use these as the required system of due process safeguards that Section 504 requires, not all states or districts do this.
We believe that as implemented by most schools, an IEP affords more support for students and input for parents than a 504 Plan. By setting goals and methods for determining if these educational goals are being met, an IEP focuses not simply on access, but more broadly on educational methodology and improved performance.
School districts must supply data to their state and states, in turn, must account to the federal government for the number of students who receive IEPs. This is related to the IDEA's status as a funding law and is designed to make sure that districts aren't classifying too many students or too many students of a particular background, as requiring IEPs. There is thus some pressure on some districts to limit the number of students with IEPs. Such data is not required in the same way for 504 Plans, so some districts may be more comfortable offering a 504 Plan in lieu of an IEP.
When appropriate, such as for a student with a medical condition, or with ADHD and without any concurrent learning difficulties, or for a student who only needs accommodations, such as extended time on exams, Section 504 can be an important and effective law. But for students whose issues are more complex and include substantial learning challenges requiring special educational services and supports, we urge parents to seek an IEP as the best way to obtain what will best serve their child.