There are some multilevel schools in the USA, but most are not.  Public school models can vary from school district to school district.  Form example I'm in a very large district.  There is an elementary school that has looping.  What that means is that students stay with the same teacher for two years K-1, 2-3, 4-5.  We also have two school that follow a Montessori model.  They often have several ages in one classroom.  Other schools have special program where only a few classrooms participate.  Some charter and private schools also have multiage classrooms.

If you are looking for a school in a particular area, use Education.com SchoolFinder link.  You can find and compare schools in a particular area.  For more information about multiage programs, you could then contact the school directly.