From what All Might said, he was only going to teach him how to attack with Air Pressure until he reached 15%. But Izuku went beyond his expectations and was able to brute force his way with 20% and he wasn't breaking any bones. So he decided it was worth to teach him then.
SBA: So yes they will fight.
Koichi has superior mobility in the air, has higher AP, better reflexes via instinctive reactions, superior ranged attacks, along with the battlefield advantage. Izuku has 8-B durability, can boost his speed/power with Fa Jin/Blackwhip, can detect danger, can block off sight with smokescreen, and can restrain with Blackwhip via higher lifting strength.
Izuku restraining Koichi with Blackwhip would be a quick victory. Since he has no way of breaking out of it, as his LS is Unknown while Izuku's is Class 10. However Koichi is a master at dodging and avoiding attacks, so while the idea would be quick, actually restraining him would be difficult.
When it comes to a fight at ranged. I'd honestly say they're inconclusive, since both have advantages in order to avoid each others attacks. Koichi's reactions and Izuku's Danger Sense. While Koichi has a homefield advantage, sneak attacks won't really do much thanks to Danger Sense. And Izuku can also make a quick retreat with Smokescreen and Fa Jin if he needs to take a breather for whatever reason.
Smokescreen completely stops Koichi's attacks, and it covers a lot of distance. At any point Izuku can stop his attacks and catch his breath if he needs to.
Koichi ranged attacks are even less threatening than Nine's, who's attacks were just as numerous and could home in on him. He's used to dealing with people who spam ranged attacks, and now he has more abilities. Also Koichi will hold back, while SBA makes it so he won't give up it doesn't change that he won't go full power.
He still won't see Izuku worse than Number 6, which means he won't throw any lethal attacks. Meaning his attacks won't be as fast and strong as they could be. Since even with Number 6 the only reason he did use his full power was because destroying his plasma body wasn't actually damaging to him.
Blackwhip + Fa Jin will be difficult. Since Izuku's combination of those moves can increase his power and speed drastically. With a slingshot he can blitz people on par with himself, though Koichi's reactions may help. I don't believe they'd be enough for the first attack. Though once he understands the attack, they'll be able to help him avoid it in advance as long as he can see him before he launches. And Koichi's ranged attacks are superior, as he can actually attack with energy blasts, while Izuku's only ranged option is trying to grab him with Blackwhip.
I don't believe Koichi tiring out Izuku is more likely than him tiring out himself. Assuming Koichi can avoid Blackwhip, I don't think he can take out Izuku in any quick fashion. And I believe Izuku's stamina and higher pain tolerance will give him the win in the end.
Koichi has no reason to get close and isn't a close up fighter to begin with. While Izuku is better up close, Koichi's ranged attacks aren't anything special that he can't deal with. Both of them can avoid attacks from other, though Izuku has a good chance of landing a nasty blow with Fa Jin + Blackwhip. As long as he plays his cards right and notice how good Koichi is at avoiding. Using a combination of smokescreen and blocking line of sight will help him land a big blow.
I can see Izuku wearing Koichi out with his Fa Jin + Blackwhip attacks, since we have no reason to believe Koichi can react to attacks he doesn't see coming. And Izuku will likely only attack from blind spots, making sure Koichi can't seem him when he is about to launch. However it'll be hard, since Koichi knows the area better than he does. That is why I believe the fight will come down to stamina. And I give my vote to Izuku via better endurance over a long period of time.
However I think this is very close, and admit that maybe I'm missing something.