He wasn't top of his class, but he was near it. Keigo got upset because Ichigo wasnt in the low ranks like him, which surprised him because Ichigo barely paid attention even before becoming a shinigami and being distracted with saving lives and whatnot. Pretty sure Ishida was top of the class, they show it in the episode he's introduced. Isshin never rode his ass, he even states he has a hands-off approach to parenting (but not with the girls lol). I think the school grades bit shows that he can understand things without really trying, so if he did focus and put his mind to it, he could easily.
Without a doubt his intelligence is nowhere NEAR the people you mentioned - Ishida, Urahara, Aizen etc. But hes smarter (intelligent....er) than the average bear.
Also, to me it seemed he was able to control his spiritual energy in that cannon ball, it was just too much for him to be able to lower the amount of power. (sorry for ref another manga) but it's kinda like Soul Eater, where blackstar's wavelength was too much for the other two - it's too big for him to control with such finesse. But i do agree that he lacks reiatsu control in general.