That's obviously false. If you don't pay attention or don't do your homework or skip classes eventually you will fall back enough not to understand what is being taught.
Teacher can't repeat basics forever because there are other students in the class too, maybe they do pay attention and do their homework and would like to learn - wasting their time because one person doesn't care is absurd.
BTW I think online remote learning (which was implemented in my country because of COVID-19) could help with this issue. There's no reason for kids to be divided into classes according only to their physical location. It's possible to organize classes from all over the country divided by results of some placement tests, where every student has problems with the exact same concepts.
This shouldn't completely replace normal school, but it could be useful tool for keeping struggling kids up to speed (let's say there's 1 hour a week of remote math class where every kid is put into appropriate countrywide group of kids with similar problems).