In order for a student to receive a 504 Plan, four things must get done; referral, determine eligibility, plan development , and monitoring and adapting.
A number of people can request a referral to a 504 Plan, including parent(s), teachers, physician, and therapist. Once a referral is obtained the actual process of determining eligibility begins.
A student is eligible if one of three things must be met: "(1) have a physical or mental impairment that substantially limits one or more major life activities; or (2) have a record of such an impairment; or (3) be regarded as having such an impairment" (Protecting students with disabilities) For most of these all it takes is a note from a physician or therapist, possibly combined with some observations of the student. If a student meets one of these three criteria, then they are eligible and can begin developing a plan for them.
Plans are developed by any one who is familiar with the student and/or the school itself. This can include teachers, parents, social workers, psychologists, therapists etc. Once a plan is developed it can be implemented, parental approval is NOT required.
After a plan is implemented, students are monitored to ensure the accommodations are appropriate for them. Plans can be revised annually.