Hi:
An ADHD diagnosis alone does not legally compel a school to offer an IEP. The school assessment must show that the child’s ADHD significantly limits his or her ability to perform well academically. That is obviously subjective (you can learn more about this criteria here: https://www.additudemag.com/special-education-accommodations/). And you absolutely have recourse if your child’s school is denying an IEP; this article does a great job of describing how to contest that decision: https://www.additudemag.com/iep-step-6-if-you-disagree-with-the-schools-assessment/
Best of luck!