Honestly his performance in the war etc and fighting pain, Tsunade could never let him do the chuunin exams.. he'd demolish every one thus causing noone to go up that year... he'll just be given a promotion .. he is stronger than the hokage after all ..True xd
But in the other hand... i think he wouldn't have any trouble to complete all the exams anyway, haha.
yeah. chuunin exams in the early chapters were cancelled. but tsunade made shikamaru a chuunin.I Don't believe it was ever stated that to become hokage you must go through the ranks...
You just need to be suited for the position - aka strongest in the village and smart enough to run it