I want a character who is disliked by the school for being a condescending prick!

Literally the most annoying person ever, they're in the accelerated program and constantly need to remain people of that, and they're such an asshole about it too. They talk to anyone who's not the advanced program as if they were a two year old learning about colors, saying stuff like: "Big boy/girl words" "This is a Pencil. Can You Say “Pencil”" "Good job!" "You're learning already"

Nobody talks to him but he's constantly talking to people to remind them that he's in the accelerated program and he's smart

You could be literally at your desk doing your homework, minding your own business, and then this person appears and asks you why are you doing homework from weeks ago

"This is not homework from weeks ago, this was assigned today"

"Oh yeah, my bad. I forgot we were in the slow class, and everyone here is doing stuff us accelerated kids already did"

"..."

"Sorry, I keep forgetting not everyone's in the accelerated class like me"

"Nobody cares, Charlie!"

And it doesn't just stop there, because when someone is better than them. They go full on 20 questions mode, because apparently the only possible explanation for someone being better than them is "spoiled" or "has some close relationship with the teacher"

And when they say something that's debatable, then the debate is over. They're right, and you must agree with them, everybody has to understand that and tell them they're right. No debates, no "technically", no proof that contradicts their claim will be accepted. They're right

Technically the term "nerd" refers to someone who's socially awkward but skilled in some area yet no where it's stated that area gotta be numbers or computers. Artists are usually depicted weird but not necessarily socially awkward more in their own bubble. Not in the way nerds typically are. Then ofc every jock gotta be stupid for some reason and an ass cause you can't enjoy & be good at sports without being a tyrant. Now imagine a socially awkward jock! Barely see history interested otherwise it's just an extra for the math nerd. Musicians usually are "the prodigy" but not really what you define a nerd. Or hey image a musician who isn't a prodigy but does their best

Basically your local SMART character talks in numbers. Everyone else can be damned, they aren't smart

Quinton Reviews made a lot of long videos with a lot of good jokes,

Have you ever seen the trope of the nerd/“smart” person questioning their intelligence/capabilities and their insecurity gets addressed?

If so, who are they, and what piece of media are they from?

I am sure he doesn`t like school.

Not in a `I am too smart for this mundane level of education` pretentious manner, just in a normal kid way. He simply finds it... not as amusing as other activities.

His grades are not as good as people assume them to be. 

There are principally two reasons for this: 

He is very selective about the stuff he puts effort in. 

All the time he spends investigating the digiworld or using/updating his computer are hours he doesn`t invest on studying. The results need to show that. Honestly, I don`t see Kou as someone who could get straight As without trying. 

He probably still obtains `better than average´ grades, tho. Just not perfect, as in Ken Ichijouji tier of prodigy kid. His bests are  maths and physics because he is very interested in numbers and therefore doesn`t mind working hard to obtain the best mark possible. I also headcanon Izzy as a grammar nazi, so he would be good at that formal and structural part of Japanesse and any other language he is enrolled in.

He is pretty bad at Literature and History, which are always his worst subjects, the ones that lower his general score.

Koushiro is not a bookworm.

I headcanon Sora, Takeru, Hikari and even Yamato being more into reading than him. I don`t include Jyou because I think he is too busy with academic bullshit to do books for leisure; however, if he had time he would also be a greater book fan than Izzy.

He does like sports. 

If he doesn`t  practice them is due to the demanding routine and healthy habits they require; not because he hates or absolutely sucks at them. 

In fact, Kou always joins the `let`s go watch the -insert super important  encounter of any sport- at my house´ kind of plans and rarely misses Taichi`s games.

This last one is not only a `friend compromise´ thing, he genuenly enjoys watching.

On top of that, Izzy would most likely make money betting.

He has a bad temper when needed and is a very critical boy. 

I believe a good canon example is when he snaps at Mimi, his crush in Digimon Tri . A stereotypically adorkable guy would just droll over her and do whatever she wants without questioning it. Not Kou and I am soo proud of him for it.