You... what? You can't be serious. Simply putting "exercise and adhd" into Google® got me multiple studies which say that exercise helps, along with many other websites (some reputable and others possibly not) all saying essentially the same thing. You can easily find licensed physicians giving this same advice to their patients, and they observe that it works. And, off the top of my head, exercising elevates dopamine and some other neurotransmitters, which are well-known to be deficient in people with ADHD. That's one of the reasons the drugs work: stimulants raise the levels of those neurotransmitters. Personally speaking, I've found that
any activity or action that raises my dopamine level has the same effect on my executive function as a stimulant drug, although this can be a very short-lived improvement (worst case, it comes and goes entirely within ~5 minutes).
You may also consider this from an evolutionary angle. Mammals such as us are not meant to sit inside at a desk all day (whether that's school or any other menial work), so it's no surprize that doing that alters and impairs development versus the conditions we evolved to live in. It's really easy to encounter the biological equivalent of undefined behaviors and invalid state in modernity. If you don't know what I mean, here's an easier to see example: It's well-known in recent years that myopia (shortsightedness) is predominantly caused by insufficient exposure to sunlight as a child. As an aside, I find it very interesting that the reason sunlight exposure matters is it elevates dopamine in the retinas, and eyes with dopamine deficiency don't develop correctly.
Another angle is mental exercize: It's more difficult to physically exercize with ADHD, but it's possible, so mentally pushing past the difficulty (or, more likely, having someone encourage you and help you) can train you to do that with other things and teach you how to self-manage your symptoms with mental decisions / willpower. This is a pretty poor solution on its own, but it's a great force multiplier when combined with other strategies.