Greetings, I am definitely Oregon Coast, so welcome to the area, but I am a little farther from Coos Bay than the campgrounds you mention. And my bus is not operational yet, so I'm not that aware of schoolie friendly places. I usually go camping mostly in the north end of the Siskiyou National Forest, but that is quite a ways from Coos Bay, at least 70 miles, but it's beautiful there. You will be able to find boondocking sites up Coos River, but I don't really know how far it is. We used to go camping up Coos River when I was in college, but I haven't been up there for decades. And there's no National Forest up there, so no dispersed camping.
As far as commercial campgrounds close to Coos Bay, a friend stayed at that campground near the Fred Meyer store last year while doing chemotherapy in Coos Bay, but he did not have a schoolie. He had a BlueBird WanderLodge, so I don't know if they let in schoolies or not. That's all I know about commercial campgrounds in the area. Oh, and it's not really a campground, but they do let people overnight in Sturdivant Park in Coquille for $10 per night. But there are no hookups of any type, just the park rest rooms, and the park is nearly 20 miles from Coos Bay.
Sorry I can't be more helpful, good luck with your search.