Education is a very fluid idea and a very fluid concept that has changed many times throughout history. The way we teach our students changes based on what we consider relevant at the time.
Before the time of larger cities like New York and Chicago in America (1800's) schools were filled with students of any age all in the same room, including different levels of intelligence, maturity, etc. This was needed as there were few teachers and not enough students in small towns to warrant larger schools. The information taught was all what was relevant to that town and its economy. If you came from a farming town, your studies revolved around farming, etc.
Eventually cities grew and so did our population of students. Around the early 1900s we had a major education reform where we began dividing our students up by age. We also borrowed the grading scale used by the meat processing industry (A,B,C,D,Failure) to grade our student's work. The method of grouping students by age and moving them through "grades" was borrowed from Henry Ford's production lines that revolutionized the car making industry. While efficient, not all students at the same age come from the same level of intellect or same background. We continue to use these systems as it is the most efficient, though it has its own problems. During this time period we also had very little understanding of individual learning styles and mental disorders, such as learning deficiencies. Very little work was done to bring in those having a harder time in the classroom and all students would be given similar lessons and tests as it was believed all students learned about the same way. We also still see all content being centralized around the dominant needs of the area. If you lived in a farming state, the state standards revolved around farming. Those in Kansas learned about crops, those in New York learned about taxes and money.
In our current day and age, we recognize that not all students stay and live their lives out where they were raised. Movement around the country is common, and we began to see that not all systems of education (standards) were so easily moved. A student in Kansas who can measure in cords of wood and rope would have a difficult time living in New York competing with students who learned a New York education. This led to the creation of the Common Core initiative. The Common Core is a set of standards adopted by almost every state in the country that they use to teach their students. This way, a student can move from State to State and hopefully their education wouldn't be very different. It also readies students to have the same chances at getting a job or other position no matter where they move to.
This is far from the end of how we see education, as it will be always changing and adapting to fit the needs of our students and of our general populace. Each year we move progressively more toward an educational experience that is better suited towards the individuals, rather than the "teach-the-masses" methods employed by out fore bearers.