If you mean public schools, paid by taxpayers, and free to students, that would be known as K – 12, ages 5 – 18. They are roughly divided by age groups as stated by @DominicX above.
There are also public classes for some children below the age of 5, called Head Start, which is only available to children that are labeled underprivileged. Most pre-schools are fee based, and the parents have to pay for them.
There are public (taxpayer funded) classes for students over the age of 18 who aren’t able to compete their graduation requirements, sometimes called Continuation School. It can be confusing, because Continuation School also provides classes for students who have been expelled from the regular schools for mis-behavior.
As if that isn’t enough, there are many levels of private, fee based, schools. Usually the parents of the students pay the tuition for these schools. They range from special interest school, like Montessori and Waldorf schools to Military Academies and parochial schools like those sponsored by the Catholic, Jewish, and Muslim religions, to the various homeschools, which are run like independent private schools by the parents.