As you write lesson plans, you will need to refer to standards for your subject area. Standards are created to ensure that students from one classroom to another are taught the same basic information in a particular subject. While that concept might seem simple stated as such, it can in fact be much more complicated for the individual classroom teacher.
Each state develops its own standards according to their own system. This creates a system whereby a tenth-grader who moves from Texas to Florida half-way through the school year will be faced with quite a different curriculum and standards that need to be met.
The situation is further complicated by the periodic changes that occur to standards. When a particular curriculum area meets to change their standards, teachers are handed and expected to teach to a new group of standards from that point on. This can cause problems when drastic changes occur and teachers are still using textbooks based on the older standards.
So why does this situation exist? The answer lies in flexibility and the desire for local control. States are able to determine what is important for their citizens and focus the curriculum accordingly.
There are no "official" national standards that teachers and schools must follow. Further, a simple search on the Internet will reveal that even within a single subject area there are often a number of available national standards created by multiple organizations. Therefore, the status today for national standards is to enhance and inform current use of state standards. With this said, the rise in acceptance of common core standards points to a future where more states and subjects fall under the umbrella of national standards.
Will there ever be mandated national standards? At this time, it looks doubtful. Proponents claim that curriculum would be standardized across the nation. However, the desire for local control is one of the foundational beliefs of the United States. An individual focus desired by the states would be virtually impossible with national standards.