History is hard to teach well; there is way too much of it to instruct in any amount of time. You could probably fill an entire semester with nothing other than the causes of World War I, for example. You thus need to ask yourself why you're teaching history in the first place.
In my school system, we taught history from 4th grade onwards as state history, world history, US history (for 2 years), civics, skip (•), world history, US history, and finally US government. World history usually hits Mesopotamia, Egypt, Greece, Rome, Crusades, Reformation, Renaissance, Age of Discovery, maybe ancient China and India. I'm also pretty sure we somehow got a tiny bit of Mesoamerica squashed in there (although it was little more than "here's Olmecs and Maya and Aztecs, and there was this thing called Inca in the Andes"). You'll notice that everything after the founding of the US is pretty much ignored in world history, so you'll learn that (barring electives) mostly when you talk about it in the context of US history. Even though I often complain that "world" history usually just means European history, you'll also notice that an awful lot of European history is skipped. No discussion of major political events like the War of Spanish Succession, the Thirty Years' War, or the Spanish Reconquista (except as maybe a throwaway like "Spain was recently united and reconquered from the Muslims when Christopher Columbus came to court...").
Given the problems with history already, I don't think there is much hope for being able to give African history any decent coverage. Quite frankly, history classes are mostly geared towards nationalism (hopefully more about positive national pride and not about jingoism and ideas of national superiority) and focusing on foundations of Western thought, neither of which really benefit from learning about Africa. That said, it's probably worth expanding the unit on colonization to include brief histories of the colonized polities prior to colonization. For example, touch on Timbuktu's and Djenné's roles as major centers of Islamic learning in the Early Modern. A full picture of Africa is probably impossible to impart, but effort should be taken to avoid thinking of it only as lands just waiting to be colonized by Europeans.
• I went to a magnet school which had slightly different scheduling than other schools in the county. I think it's supposed to be normally two years of world history, but the course in 9th grade was replaced with a technology course and you'd be expected to grab another credit of history via some elective... which is how I got a course that was history of art, architecture, and technology.