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.