I think it's fair to say that schoolwork just outpaces attention span and focus at some point. Many people do well in their first years, and the struggles often only manifest themselves when more and more subjects get added, each with higher workloads than before.

Makes a lot of sense, but some people just cling to the but you did so well in elementary school thing

You're normally expected to have lifelong symptoms, but that doesn't mean you had to do badly in school.

You can constantly daydream, lose stuff and turn up late for everything and still ace tests, at least early on. It gets harder to get away with this later in life.