There are many skills that fall under executive functioning, and it is possible to be very strong at the organizational skills you describe (staying on track of assignments, using a planner, maintaining belongings and schedule) and still have difficulty with aspects of attention and or executive functioning such as concentration, working memory, dealing with ambiguity.
I have a 17 year old dd that is EXACTLY like that. At first glance, VERY organized and on top of things. But, in 7th grade, her academic performance faltered and by 9th grade her grades had slipped substantially. She was convinced she had ADHD -in like her brother. She told me that she could rarely stay focused on lectures, could not remember all aspects of instructions, was very slow in getting her work done, often missed details in assignments, made "careless" errors etc. We had her evaluated and realized that while in some respects her EF was very strong, there were some problem areas that affected everything she did in school. Even in the testing - there were HUGE variations in her performance - often indicative of an attention and concentration issue.
In addition to EF difficulties, she also received a diagnosis for ADHD - but we decided not to try medication. Instead, we hired a tutor/coach and made some environmental changes such as taking her phone away from her during HW time. This was enough for her. (we are not against medication - her brother uses stimulants)
She can still be spacey, forgetful, slow. But she does very well when she is able to use her strategies and impose her own structure.
