What is the age group that children have to be in a second floor school?

There's no specific age range that mandates children be in a single-story or two-story school. Building codes and fire safety regulations dictate the requirements for things like stairwells, fire escapes, and sprinkler systems, regardless of the age of the students. These safety measures are designed to protect children of all ages. A school's design is based on these regulations and not the ages of the children who attend.