I remember back to my high school days when I was in AP US History, and there was a large amount of information to absorb from just 238 years of this country being around. I wondered what teachers would do in a century or two's time when there was even more content to learn about, and how they would be able to fit all of it into their curriculum.
I guess that same can be said about the writers of history books. Will they just categorize things based on their level of importance and only put the more significant stuff in?
Source: I'm currently going to graduate school for my Master's and teaching certification in high school social studies. My girlfriend's an English teacher, my brother's a college psychology professor, my sister-in-law is an elementary school teacher, and I have friends throughout the field and we all agree on the basics of what I've written below. Of course, everyone is welcome to their own opinions so feel free to take the following with a grain of salt:
At a guess, it would just have to be split up. In non-AP courses, U.S. history is typically split into two sections: Jamestown to Civil War, and Reconstruction to Present. Usually, the "present" as we know it isn't reached because history is still being made so teachers can only guess at the significance of events. Most often it ends during the 1990s, though maybe the early 2000s are involved depending on the school.
As time goes on and the way classes and students are taught evolves, some things may be left out. For example, a professor of mine said he didn't think it was important if people knew who won the American Civil War. How is that information relevant to kids today? Do kids and young adults need to know who won the Civil War to be a productive member of modern society? Maybe for how the war developed and what its consequences are, but maybe not necessarily who won. You have to come up with your own opinion for that. The point is, the way history is taught is moving from informative, fact-based lessons to inquiry-based lessons. In other words, who cares if you can spew out names, dates, and places like a walking encyclopedia? What matters is what you can do with that knowledge. Can you analyze a text? See patterns in events? Realize the cyclical nature of history? That's what's important and that is, slowly, the direction history classes are taking. So maybe in the future, teachers might not focus as much on the events, but rather the consequences of events, the ideals that resulted or came before or changed as a result of events, and the development of analytical skills in relation to primary, secondary, and reliable sources.
I still think it can be helpful to know about the events and topics currently being taught, but maybe not in as much detail as it is now. I'm currently interning at a high school where the curriculum has the students take a year of U.S. history, two years of modern world history, and then an elective (civics, economics, government, psychology). The world prior to 1800 is ignored unless you take AP European, and even then it really only starts not long before the French Revolution. So already we're dismissing (at least in my state, at my school) Ancient Rome, Ancient Greece, Ancient Egypt, China (touched on during Communism unit), Japan (touched on during revolution unit), India (touched on because of revolution unit), the Aztecs, the Incas, the Mayans, the hundreds of tribes in pre-colonial America and Africa, Russia prior to its Revolution. I think topics in particular nations will only get more sparse as time goes on.
Every place has certain aspects that it chooses to focus on at the expense of others. Its anecdotally notable how knowledgeable the average American person is on US history but how lacking they can be about world history outside the US. However this is only a period of about 300 years. My understanding is that they already "cut" huge swathes of history in their continent which doesnt involve the political entity of "the USA".
Similarly, a country like Britain with a comparatively deep history including Romans, Vikings, Civil War, Tudors, Enlightenment and World Empire will not attempt to cover all or even most of those periods. Although it is not a period of history that everyone is interested in, they tend to focus on historical periods which perhaps have more relevance to the politics of today - the move from country to town, the industrial revolution, the World Wars, the end of Empire, organised labour.
Simple answer is that history courses are designed to provide an overview, not every historical detail. Every teacher is constrained by the limits of time, whether in a given lecture, or semester, or school year. They then must practice the art of concision and provide what they determine is important information about the topic. In terms of writing history, this is why historians have specializations. We write about specific themes or topics within a periodization that fits within a larger historiography. Whether in teaching or writing, there's always a time when you need to stop.
Short answer: They dont cover half of history as it is today.
Story: so semi-recently it was proposed in a nearby area to not cover the civil war (mainly for political reasons). A teacher stood up during a townhall meeting about the issue, and said "Honestly as a teacher, I really don't care if we cover it or not, we don't have time to cover it as is today. All I talk about during that chapter is the emancipation proclamation, and that Lincoln got killed. Takes me about 30 minutes, which is all I have in the day to each history."