The ADA does not define medical conditions that constitute disabilities. Instead, the ADA has a general definition of disability that each person must meet so some people with ADHD will have a disability under the ADA and some will not.
Parents of children who are struggling in school should discuss the problems with their child’s doctor and school professionals. Most children with ADHD do not require additional school interventions. Some will need a space with few distractions to complete work or more time on tests. These “accommodations” can be specified in a “504 Plan.” Some children with more extreme learning problems may have a more extensive evaluation to determine if they are ESE (Exceptional Student Education) and require an IEP (Individualized Education Plan). You will find more information about these school-based programs on the website for your state department of education.