Ok, let me leave it by saying literally everyth9ing I have read or heard about ADHD concerns disruptive behavior. Not saying you are not correct in classifying an inability to focus as ADHD, it is just that I have simply never heard of what you describe as being ADHD. Usually it is called day dreaming.
The more common term used with such kids is ADD/ADHD ... Attention Deficit Disorder/Attention Deficit Hyperactivity Disorder. Some have the H, most don’t.