Both can be true.
1. School is boring af and humans didn't evolve to sit at a desk for 7 hours learning boring crap they'll never need. I fully agree.
2. ADHD is still real and it can be (more often than not is) very debilitating.
The fact you don't have any experience of it or choose to believe it doesn't exist doesn't change reality, I'm afraid. Believe in it. Don't believe in it. Your beliefs don't change anything.
You can choose to not believe in diabetes, dyslexia, Down's Syndrome, autism and schizophrenia or any number of conditions too. But reality doesn't care about your feelings. People actually experience this shit and the fact you haven't personally experienced what they've been through is not important.
So it really is irrelevant what you think or don't think.
So believe it's a made-up diagnosis if you want. It won't change anything. And people who actually experience it will continue to experience it with or without your approval.