Yugi dislikes school. A lot. School, to him, is just somewhere he goes to meet friends, and that's that. He struggles immensely in school, in most any subject, to get decent grades. He struggles to focus, to properly finish homework, to even pay attention during lectures. It's why he's a poor student, and pretty much any good students or teachers look down on him for it.
There are multiple reasons for this, but all lead to a single result: Yugi doesn't care because no one else does.
As proven in his games, his duels, and the types of puzzles he can complete, Yugi is smart. He's an incredibly sharp kid who can pick up on and strategize quickly, and even if Yami took over for most of his life, he's still no less skilled. He's also insatiable when it comes to things he enjoys or has an interest in, devouring everything in those subjects that he can get his hands on and looking for more.
And yet, these don't show in school. There's two main reasons.
The first is that Yugi is too smart, as well as being a fairly hands-on person. Because his mind is sharp and he's used to doing complicated things (like putting together models) or thinking through complicated things (riddle books or puzzle games), Yugi has an issue with trying to do things that are "too dumb" for him. His schoolwork is too easy overall for him, which means he finds no challenge or investment in it and leads to him having an immensely hard time actually... paying attention to it. The fact that schoolbooks, schoolwork, and lectures are informative first with basically no engagement only makes it worse, since Yugi gets fidgety about things, especially since what he's being told he may already know. The result is that he often dozes or checks out during lectures, struggles to get through his work even though he knows he needs to, and just drifts over to things that can keep his attention and challenge him.
The second is because Yugi's school is awful, and with awful teachers. As shown in S0, basically the entire staff of that place is terrible, with putting down or torturing students, calling them idiots, being nasty, rude and abusive. This is only the sample directly shown in canon, but a school that has a principal who belittled and then tried to expel students for being "dumb" is not one that hires caring people, and so every teacher Yugi had in that school was a terrible teacher. Even if his "problems" were brought up, they never addressed how to fix them, only putting all the burden on him -- just try harder, get more sleep, just focus, all things that do nothing to actually help keep Yugi engaged. Because of that, and because the school does nothing about problem students (Jounouchi and Honda were not punished for bullying Yugi -- and then, when Ushio stepped up, the school did nothing to stop him, either), Yugi has absolutely no trust in his teachers, and that's made him dislike school all the more, which leads to even less effort on his part.
Yugi could be a very good student at minimum! He's smart enough and dedicated enough. But because of all the issues he's suffered and how his own needs are ignored, his issues prevent him from doing the majority of the work and his apathy prevents the rest, because "Why even bother?"
SCHOOL
There are multiple reasons for this, but all lead to a single result: Yugi doesn't care because no one else does.
As proven in his games, his duels, and the types of puzzles he can complete, Yugi is smart. He's an incredibly sharp kid who can pick up on and strategize quickly, and even if Yami took over for most of his life, he's still no less skilled. He's also insatiable when it comes to things he enjoys or has an interest in, devouring everything in those subjects that he can get his hands on and looking for more.
And yet, these don't show in school. There's two main reasons.
The first is that Yugi is too smart, as well as being a fairly hands-on person. Because his mind is sharp and he's used to doing complicated things (like putting together models) or thinking through complicated things (riddle books or puzzle games), Yugi has an issue with trying to do things that are "too dumb" for him. His schoolwork is too easy overall for him, which means he finds no challenge or investment in it and leads to him having an immensely hard time actually... paying attention to it. The fact that schoolbooks, schoolwork, and lectures are informative first with basically no engagement only makes it worse, since Yugi gets fidgety about things, especially since what he's being told he may already know. The result is that he often dozes or checks out during lectures, struggles to get through his work even though he knows he needs to, and just drifts over to things that can keep his attention and challenge him.
The second is because Yugi's school is awful, and with awful teachers. As shown in S0, basically the entire staff of that place is terrible, with putting down or torturing students, calling them idiots, being nasty, rude and abusive. This is only the sample directly shown in canon, but a school that has a principal who belittled and then tried to expel students for being "dumb" is not one that hires caring people, and so every teacher Yugi had in that school was a terrible teacher. Even if his "problems" were brought up, they never addressed how to fix them, only putting all the burden on him -- just try harder, get more sleep, just focus, all things that do nothing to actually help keep Yugi engaged. Because of that, and because the school does nothing about problem students (Jounouchi and Honda were not punished for bullying Yugi -- and then, when Ushio stepped up, the school did nothing to stop him, either), Yugi has absolutely no trust in his teachers, and that's made him dislike school all the more, which leads to even less effort on his part.
Yugi could be a very good student at minimum! He's smart enough and dedicated enough. But because of all the issues he's suffered and how his own needs are ignored, his issues prevent him from doing the majority of the work and his apathy prevents the rest, because "Why even bother?"